[apple/xnu.git] / osfmk / kern / thread_act.c

/*
 * Copyright (c) 2000 Apple Computer, Inc. All rights reserved.
 *
 * @APPLE_LICENSE_HEADER_START@
 * 
 * The contents of this file constitute Original Code as defined in and
 * are subject to the Apple Public Source License Version 1.1 (the
 * "License").  You may not use this file except in compliance with the
 * License.  Please obtain a copy of the License at
 * http://www.apple.com/publicsource and read it before using this file.
 * 
 * This Original Code and all software distributed under the License are
 * distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT.  Please see the
 * License for the specific language governing rights and limitations
 * under the License.
 * 
 * @APPLE_LICENSE_HEADER_END@
 */
/*
 * @OSF_FREE_COPYRIGHT@
 */
/*
 * Copyright (c) 1993 The University of Utah and
 * the Center for Software Science (CSS).  All rights reserved.
 *
 * Permission to use, copy, modify and distribute this software and its
 * documentation is hereby granted, provided that both the copyright
 * notice and this permission notice appear in all copies of the
 * software, derivative works or modified versions, and any portions
 * thereof, and that both notices appear in supporting documentation.
 *
 * THE UNIVERSITY OF UTAH AND CSS ALLOW FREE USE OF THIS SOFTWARE IN ITS "AS
 * IS" CONDITION.  THE UNIVERSITY OF UTAH AND CSS DISCLAIM ANY LIABILITY OF
 * ANY KIND FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
 *
 * CSS requests users of this software to return to css-dist@cs.utah.edu any
 * improvements that they make and grant CSS redistribution rights.
 *
 *      Author: Bryan Ford, University of Utah CSS
 *
 *      Thread_Activation management routines
 */

#include <cpus.h>
#include <task_swapper.h>
#include <mach/kern_return.h>
#include <mach/alert.h>
#include <kern/etap_macros.h>
#include <kern/mach_param.h>
#include <kern/zalloc.h>
#include <kern/thread.h>
#include <kern/thread_swap.h>
#include <kern/task.h>
#include <kern/task_swap.h>
#include <kern/thread_act.h>
#include <kern/thread_pool.h>
#include <kern/sched_prim.h>
#include <kern/misc_protos.h>
#include <kern/assert.h>
#include <kern/exception.h>
#include <kern/ipc_mig.h>
#include <kern/ipc_tt.h>
#include <kern/profile.h>
#include <kern/machine.h>
#include <kern/spl.h>
#include <kern/syscall_subr.h>
#include <kern/sync_lock.h>
#include <kern/sf.h>
#include <kern/mk_sp.h> /*** ??? fix so this can be removed ***/
#include <mach_prof.h>
#include <mach/rpc.h>

/*
 * Debugging printf control
 */
#if     MACH_ASSERT
unsigned int    watchacts =       0 /* WA_ALL */
                                    ;   /* Do-it-yourself & patchable */
#endif

/*
 * Track the number of times we need to swapin a thread to deallocate it.
 */
int act_free_swapin = 0;

/*
 * Forward declarations for functions local to this file.
 */
kern_return_t   act_abort( thread_act_t, int);
void            special_handler(ReturnHandler *, thread_act_t);
void            nudge(thread_act_t);
kern_return_t   act_set_state_locked(thread_act_t, int,
                                        thread_state_t,
                                        mach_msg_type_number_t);
kern_return_t   act_get_state_locked(thread_act_t, int,
                                        thread_state_t,
                                        mach_msg_type_number_t *);
void            act_set_apc(thread_act_t);
void            act_clr_apc(thread_act_t);
void            act_user_to_kernel(thread_act_t);
void            act_ulock_release_all(thread_act_t thr_act);

void            install_special_handler_locked(thread_act_t);

static zone_t   thr_act_zone;

/*
 * Thread interfaces accessed via a thread_activation:
 */


/*
 * Internal routine to terminate a thread.
 * Called with task locked.
 */
kern_return_t
thread_terminate_internal(
        register thread_act_t   thr_act)
{
        thread_t        thread;
        task_t          task;
        struct ipc_port *iplock;
        kern_return_t   ret;
#if     NCPUS > 1
        boolean_t       held;
#endif  /* NCPUS > 1 */

#if     THREAD_SWAPPER
        thread_swap_disable(thr_act);
#endif  /* THREAD_SWAPPER */

        thread = act_lock_thread(thr_act);
        if (!thr_act->active) {
                act_unlock_thread(thr_act);
                return(KERN_TERMINATED);
        }

#if     NCPUS > 1
        /* 
         * Make sure this thread enters the kernel
         */
        if (thread != current_thread()) {
                thread_hold(thr_act);
                act_unlock_thread(thr_act);

                if (!thread_stop_wait(thread)) {
                        ret = KERN_ABORTED;
                        (void)act_lock_thread(thr_act);
                        thread_release(thr_act);
                        act_unlock_thread(thr_act);
                        return (ret);
                }

                held = TRUE;
                (void)act_lock_thread(thr_act);
        } else {
                held = FALSE;
        }
#endif  /* NCPUS > 1 */

        assert(thr_act->active);
        act_disable_task_locked(thr_act);
        ret = act_abort(thr_act,FALSE);
        act_unlock_thread(thr_act);

#if     NCPUS > 1
        if (held) {
                thread_unstop(thread);
                (void)act_lock_thread(thr_act);
                thread_release(thr_act);
                act_unlock_thread(thr_act);
        }
#endif  /* NCPUS > 1 */
        return(ret);
}

/*
 * Terminate a thread.  Called with nothing locked.
 * Returns same way.
 */
kern_return_t
thread_terminate(
        register thread_act_t   thr_act)
{
        task_t          task;
        kern_return_t   ret;

        if (thr_act == THR_ACT_NULL)
                return KERN_INVALID_ARGUMENT;

        task = thr_act->task;
        if (((task == kernel_task) || (thr_act->kernel_loaded == TRUE))
            && (current_act() != thr_act)) {
                return(KERN_FAILURE);
        }

        /*
         * Take the task lock and then call the internal routine
         * that terminates a thread (it needs the task locked).
         */
        task_lock(task);
        ret = thread_terminate_internal(thr_act);
        task_unlock(task);

        /*
         * If a kernel thread is terminating itself, force an AST here.
         * Kernel threads don't normally pass through the AST checking
         * code - and all threads finish their own termination in the
         * special handler APC.
         */
        if (((thr_act->task == kernel_task) || (thr_act->kernel_loaded == TRUE))
            && (current_act() == thr_act)) {
                ast_taken(FALSE, AST_APC, 0);
                panic("thread_terminate(): returning from ast_taken() for %x kernel activation\n", thr_act);
        }

        return ret;
}

/*
 *      thread_hold:
 *
 *      Suspend execution of the specified thread.
 *      This is a recursive-style suspension of the thread, a count of
 *      suspends is maintained.
 *
 *      Called with thr_act locked "appropriately" for synchrony with
 *      RPC (see act_lock_thread()).  Returns same way.
 */
void
thread_hold(
        register thread_act_t   thr_act)
{
        if (thr_act->suspend_count++ == 0) {
                install_special_handler(thr_act);
                nudge(thr_act);
        }
}

/*
 * Decrement internal suspension count for thr_act, setting thread
 * runnable when count falls to zero.
 *
 * Called with thr_act locked "appropriately" for synchrony
 * with RPC (see act_lock_thread()).
 */
void
thread_release(
        register thread_act_t   thr_act)
{
        if( thr_act->suspend_count &&
                (--thr_act->suspend_count == 0) )
                nudge( thr_act );
}

kern_return_t
thread_suspend(
        register thread_act_t   thr_act)
{
        thread_t                thread;

        if (thr_act == THR_ACT_NULL) {
                return(KERN_INVALID_ARGUMENT);
        }
        thread = act_lock_thread(thr_act);
        if (!thr_act->active) {
                act_unlock_thread(thr_act);
                return(KERN_TERMINATED);
        }
        if (thr_act->user_stop_count++ == 0 &&
                thr_act->suspend_count++ == 0 ) {
                install_special_handler(thr_act);
                if (thread &&
                        thr_act == thread->top_act && thread != current_thread()) {
                        nudge(thr_act);
                        act_unlock_thread(thr_act);
                        (void)thread_wait(thread);
                }
                else {
                        /*
                         * No need to wait for target thread
                         */
                        act_unlock_thread(thr_act);
                }
        }
        else {
                /*
                 * Thread is already suspended
                 */
                act_unlock_thread(thr_act);
        }
        return(KERN_SUCCESS);
}

kern_return_t
thread_resume(
        register thread_act_t   thr_act)
{
        register kern_return_t  ret;
        spl_t                   s;
        thread_t                thread;

        if (thr_act == THR_ACT_NULL)
                return(KERN_INVALID_ARGUMENT);
        thread = act_lock_thread(thr_act);
        ret = KERN_SUCCESS;

        if (thr_act->active) {
                if (thr_act->user_stop_count > 0) {
                        if( --thr_act->user_stop_count == 0 ) {
                                --thr_act->suspend_count;
                                nudge( thr_act );
                        }
                }
                else
                        ret = KERN_FAILURE;
        }
        else
                ret = KERN_TERMINATED;
        act_unlock_thread( thr_act );
        return ret;
}

/* 
 * This routine walks toward the head of an RPC chain starting at
 * a specified thread activation. An alert bit is set and a special 
 * handler is installed for each thread it encounters.
 *
 * The target thread act and thread shuttle are already locked.
 */
kern_return_t
post_alert( 
        register thread_act_t   thr_act,
        unsigned                alert_bits )
{
        thread_act_t    next;
        thread_t        thread;

        /* 
         * Chase the chain, setting alert bits and installing
         * special handlers for each thread act.
         */
        /*** Not yet SMP safe ***/
        /*** Worse, where's the activation locking as the chain is walked? ***/
        for (next = thr_act; next != THR_ACT_NULL; next = next->higher) {
                next->alerts |= alert_bits;
                install_special_handler_locked(next);
        }

        return(KERN_SUCCESS);
}

/*
 *      thread_depress_abort:
 *
 *      Prematurely abort priority depression if there is one.
 */
kern_return_t
thread_depress_abort(
        register thread_act_t   thr_act)
{
    register thread_t           thread;
        kern_return_t                   result;
    sched_policy_t                      *policy;
    spl_t                                       s;

    if (thr_act == THR_ACT_NULL)
                return (KERN_INVALID_ARGUMENT);

    thread = act_lock_thread(thr_act);
    /* if activation is terminating, this operation is not meaningful */
    if (!thr_act->active) {
                act_unlock_thread(thr_act);

                return (KERN_TERMINATED);
    }

    s = splsched();
    thread_lock(thread);
    policy = &sched_policy[thread->policy];
    thread_unlock(thread);
    splx(s);

    result = policy->sp_ops.sp_thread_depress_abort(policy, thread);

    act_unlock_thread(thr_act);

        return (result);
}


/*
 * Already locked: all RPC-related locks for thr_act (see
 * act_lock_thread()).
 */
kern_return_t
act_abort( thread_act_t thr_act, int chain_break )
{
        spl_t                   spl;
        thread_t                thread;
        struct ipc_port         *iplock = thr_act->pool_port;
        thread_act_t            orphan;
        kern_return_t           kr;
        etap_data_t             probe_data;

        ETAP_DATA_LOAD(probe_data[0], thr_act);
        ETAP_DATA_LOAD(probe_data[1], thr_act->thread);
        ETAP_PROBE_DATA(ETAP_P_ACT_ABORT,
                        0,
                        current_thread(),
                        &probe_data,
                        ETAP_DATA_ENTRY*2);

        /*
         *  If the target thread activation is not the head... 
         */
        if ( thr_act->thread->top_act != thr_act ) {
                /*
                 * mark the activation for abort,
                 * update the suspend count,
                 * always install the special handler
                 */
                install_special_handler(thr_act);

#ifdef AGRESSIVE_ABORT
                /* release state buffer for target's outstanding invocation */
                if (unwind_invoke_state(thr_act) != KERN_SUCCESS) {
                        panic("unwind_invoke_state failure");
                }

                /* release state buffer for target's incoming invocation */
                if (thr_act->lower != THR_ACT_NULL) {
                        if (unwind_invoke_state(thr_act->lower)
                            != KERN_SUCCESS) {
                                panic("unwind_invoke_state failure");
                        }
                }

                /* unlink target thread activation from shuttle chain */
                if ( thr_act->lower == THR_ACT_NULL ) {
                        /*
                         * This is the root thread activation of the chain.
                         * Unlink the root thread act from the bottom of
                         * the chain.
                         */
                        thr_act->higher->lower = THR_ACT_NULL;
                } else {
                        /*
                         * This thread act is in the middle of the chain.
                         * Unlink the thread act from the middle of the chain.
                         */
                        thr_act->higher->lower = thr_act->lower;
                        thr_act->lower->higher = thr_act->higher;

                        /* set the terminated bit for RPC return processing */
                        thr_act->lower->alerts |= SERVER_TERMINATED;
                }

                orphan = thr_act->higher;

                /* remove the activation from its thread pool */
                /* (note: this is okay for "rooted threads," too) */
                act_locked_act_set_thread_pool(thr_act, IP_NULL);

                /* (just to be thorough) release the IP lock */
                if (iplock != IP_NULL) ip_unlock(iplock);

                /* release one more reference for a rooted thread */
                if (iplock == IP_NULL) act_locked_act_deallocate(thr_act);

                /* Presumably, the only reference to this activation is
                 * now held by the caller of this routine. */
                assert(thr_act->ref_count == 1);
#else /*AGRESSIVE_ABORT*/
                /* If there is a lower activation in the RPC chain... */
                if (thr_act->lower != THR_ACT_NULL) {
                        /* ...indicate the server activation was terminated */
                        thr_act->lower->alerts |= SERVER_TERMINATED;
                }
                /* Mark (and process) any orphaned activations */
                orphan = thr_act->higher;
#endif /*AGRESSIVE_ABORT*/

                /* indicate client of orphaned chain has been terminated */
                orphan->alerts |= CLIENT_TERMINATED;

                /* 
                 * Set up posting of alert to headward portion of
                 * the RPC chain.
                 */
                /*** fix me -- orphan act is not locked ***/
                post_alert(orphan, ORPHANED);

                /*
                 * Get attention of head of RPC chain.
                 */
                nudge(thr_act->thread->top_act);
                return (KERN_SUCCESS);
        }

        /*
         * If the target thread is the end of the chain, the thread
         * has to be marked for abort and rip it out of any wait.
         */
        spl = splsched();
        thread_lock(thr_act->thread);
        if (thr_act->thread->top_act == thr_act) {
            thr_act->thread->state |= TH_ABORT;
            if (thr_act->thread->state & TH_ABORT)
                clear_wait_internal(thr_act->thread, THREAD_INTERRUPTED);
            thread_unlock(thr_act->thread);
            splx(spl);
            install_special_handler(thr_act);
            nudge( thr_act );
        }
        return KERN_SUCCESS;
}
        
kern_return_t
thread_abort(
        register thread_act_t   thr_act)
{
        int             ret;
        thread_t                thread;

        if (thr_act == THR_ACT_NULL || thr_act == current_act())
                return (KERN_INVALID_ARGUMENT);
        /*
         *      Lock the target thread and the current thread now,
         *      in case thread_halt() ends up being called below.
         */
        thread = act_lock_thread(thr_act);
        if (!thr_act->active) {
                act_unlock_thread(thr_act);
                return(KERN_TERMINATED);
        }

        ret = act_abort( thr_act, FALSE );
        act_unlock_thread( thr_act );
        return ret;
}

kern_return_t
thread_abort_safely(
        register thread_act_t   thr_act)
{
        thread_t                thread;
        spl_t                   s;

        if (thr_act == THR_ACT_NULL || thr_act == current_act())
                return(KERN_INVALID_ARGUMENT);

        thread = act_lock_thread(thr_act);
        if (!thr_act->active) {
                act_unlock_thread(thr_act);
                return(KERN_TERMINATED);
        }
        if (thread->top_act != thr_act) {
                act_unlock_thread(thr_act);
                return(KERN_FAILURE);
        }
        s = splsched();
        thread_lock(thread);

        if ( thread->at_safe_point ) {
                /*
                 * It's an abortable wait, clear it, then
                 * let the thread go and return successfully.
                 */
                clear_wait_internal(thread, THREAD_INTERRUPTED);
                thread_unlock(thread);
                act_unlock_thread(thr_act);
                splx(s);
                return KERN_SUCCESS;
        }

        /*
         * if not stopped at a safepoint, just let it go and return failure.
         */
        thread_unlock(thread);
        act_unlock_thread(thr_act);
        splx(s);
        return KERN_FAILURE;
}

/*** backward compatibility hacks ***/
#include <mach/thread_info.h>
#include <mach/thread_special_ports.h>
#include <ipc/ipc_port.h>
#include <mach/thread_act_server.h>

kern_return_t
thread_info(
        thread_act_t                    thr_act,
        thread_flavor_t                 flavor,
        thread_info_t                   thread_info_out,
        mach_msg_type_number_t  *thread_info_count)
{
        register thread_t               thread;
        kern_return_t                   result;

        if (thr_act == THR_ACT_NULL)
                return (KERN_INVALID_ARGUMENT);

        thread = act_lock_thread(thr_act);
        if (!thr_act->active) {
                act_unlock_thread(thr_act);

                return (KERN_TERMINATED);
        }

        result = thread_info_shuttle(thr_act, flavor,
                                        thread_info_out, thread_info_count);

        act_unlock_thread(thr_act);

        return (result);
}

/*
 *      Routine:        thread_get_special_port [kernel call]
 *      Purpose:
 *              Clones a send right for one of the thread's
 *              special ports.
 *      Conditions:
 *              Nothing locked.
 *      Returns:
 *              KERN_SUCCESS            Extracted a send right.
 *              KERN_INVALID_ARGUMENT   The thread is null.
 *              KERN_FAILURE            The thread is dead.
 *              KERN_INVALID_ARGUMENT   Invalid special port.
 */

kern_return_t
thread_get_special_port(
        thread_act_t    thr_act,
        int             which,
        ipc_port_t      *portp)
{
        ipc_port_t      *whichp;
        ipc_port_t      port;
        thread_t        thread;

#if     MACH_ASSERT
        if (watchacts & WA_PORT)
            printf("thread_get_special_port(thr_act=%x, which=%x port@%x=%x\n",
                thr_act, which, portp, (portp ? *portp : 0));
#endif  /* MACH_ASSERT */

        if (!thr_act)
                return KERN_INVALID_ARGUMENT;
        thread = act_lock_thread(thr_act);
        switch (which) {
                case THREAD_KERNEL_PORT:
                        whichp = &thr_act->ith_sself;
                        break;

                default:
                        act_unlock_thread(thr_act);
                        return KERN_INVALID_ARGUMENT;
        }

        if (!thr_act->active) {
                act_unlock_thread(thr_act);
                return KERN_FAILURE;
        }

        port = ipc_port_copy_send(*whichp);
        act_unlock_thread(thr_act);

        *portp = port;
        return KERN_SUCCESS;
}

/*
 *      Routine:        thread_set_special_port [kernel call]
 *      Purpose:
 *              Changes one of the thread's special ports,
 *              setting it to the supplied send right.
 *      Conditions:
 *              Nothing locked.  If successful, consumes
 *              the supplied send right.
 *      Returns:
 *              KERN_SUCCESS            Changed the special port.
 *              KERN_INVALID_ARGUMENT   The thread is null.
 *              KERN_FAILURE            The thread is dead.
 *              KERN_INVALID_ARGUMENT   Invalid special port.
 */

kern_return_t
thread_set_special_port(
        thread_act_t    thr_act,
        int             which,
        ipc_port_t      port)
{
        ipc_port_t      *whichp;
        ipc_port_t      old;
        thread_t        thread;

#if     MACH_ASSERT
        if (watchacts & WA_PORT)
                printf("thread_set_special_port(thr_act=%x,which=%x,port=%x\n",
                        thr_act, which, port);
#endif  /* MACH_ASSERT */

        if (thr_act == 0)
                return KERN_INVALID_ARGUMENT;

        thread = act_lock_thread(thr_act);
        switch (which) {
                case THREAD_KERNEL_PORT:
                        whichp = &thr_act->ith_self;
                        break;

                default:
                        act_unlock_thread(thr_act);
                        return KERN_INVALID_ARGUMENT;
        }

        if (!thr_act->active) {
                act_unlock_thread(thr_act);
                return KERN_FAILURE;
        }

        old = *whichp;
        *whichp = port;
        act_unlock_thread(thr_act);

        if (IP_VALID(old))
                ipc_port_release_send(old);
        return KERN_SUCCESS;
}

/*
 *  thread state should always be accessible by locking the thread
 *  and copying it.  The activation messes things up so for right
 *  now if it's not the top of the chain, use a special handler to
 *  get the information when the shuttle returns to the activation.
 */
kern_return_t
thread_get_state(
        register thread_act_t   thr_act,
        int                     flavor,
        thread_state_t          state,  /* pointer to OUT array */
        mach_msg_type_number_t  *state_count)   /*IN/OUT*/
{
        kern_return_t           ret;
        thread_t                thread, nthread;

#if 0 /* Grenoble - why?? */
        if (thr_act == THR_ACT_NULL || thr_act == current_act())
#else
        if (thr_act == THR_ACT_NULL)
#endif
                return (KERN_INVALID_ARGUMENT);

        thread = act_lock_thread(thr_act);
        if (!thr_act->active) {
                act_unlock_thread(thr_act);
                return(KERN_TERMINATED);
        }

        thread_hold(thr_act);
        while (1) {
                if (!thread || thr_act != thread->top_act)
                        break;
                act_unlock_thread(thr_act);
                (void)thread_stop_wait(thread);
                nthread = act_lock_thread(thr_act);
                if (nthread == thread)
                        break;
                thread_unstop(thread);
                thread = nthread;
        }
        ret = act_machine_get_state(thr_act, flavor,
                                        state, state_count);
        if (thread && thr_act == thread->top_act)
            thread_unstop(thread);
        thread_release(thr_act);
        act_unlock_thread(thr_act);

        return(ret);
}

/*
 *      Change thread's machine-dependent state.  Called with nothing
 *      locked.  Returns same way.
 */
kern_return_t
thread_set_state(
        register thread_act_t   thr_act,
        int                     flavor,
        thread_state_t          state,
        mach_msg_type_number_t  state_count)
{
        kern_return_t           ret;
        thread_t                thread, nthread;

#if 0 /* Grenoble - why?? */
        if (thr_act == THR_ACT_NULL || thr_act == current_act())
#else
        if (thr_act == THR_ACT_NULL)
#endif
                return (KERN_INVALID_ARGUMENT);
        /*
         * We have no kernel activations, so Utah's MO fails for signals etc.
         *
         * If we're blocked in the kernel, use non-blocking method, else
         * pass locked thr_act+thread in to "normal" act_[gs]et_state().
         */

        thread = act_lock_thread(thr_act);
        if (!thr_act->active) {
                act_unlock_thread(thr_act);
                return(KERN_TERMINATED);
        }

        thread_hold(thr_act);
        while (1) {
                if (!thread || thr_act != thread->top_act)
                        break;
                act_unlock_thread(thr_act);
                (void)thread_stop_wait(thread);
                nthread = act_lock_thread(thr_act);
                if (nthread == thread)
                        break;
                thread_unstop(thread);
                thread = nthread;
        }
        ret = act_machine_set_state(thr_act, flavor,
                                        state, state_count);
        if (thread && thr_act == thread->top_act)
            thread_unstop(thread);
        thread_release(thr_act);
        act_unlock_thread(thr_act);

        return(ret);
}

/*
 * Kernel-internal "thread" interfaces used outside this file:
 */

kern_return_t
thread_dup(
        thread_act_t source_thr_act, 
        thread_act_t target_thr_act)
{
        kern_return_t           ret;
        thread_t                thread, nthread;

        if (target_thr_act == THR_ACT_NULL || target_thr_act == current_act())
                return (KERN_INVALID_ARGUMENT);

        thread = act_lock_thread(target_thr_act);
        if (!target_thr_act->active) {
                act_unlock_thread(target_thr_act);
                return(KERN_TERMINATED);
        }

        thread_hold(target_thr_act);
        while (1) {
                if (!thread || target_thr_act != thread->top_act)
                        break;
                act_unlock_thread(target_thr_act);
                (void)thread_stop_wait(thread);
                nthread = act_lock_thread(target_thr_act);
                if (nthread == thread)
                        break;
                thread_unstop(thread);
                thread = nthread;
        }
        ret = act_thread_dup(source_thr_act, target_thr_act);
        if (thread && target_thr_act == thread->top_act)
            thread_unstop(thread);
        thread_release(target_thr_act);
        act_unlock_thread(target_thr_act);

        return(ret);
}


/*
 *      thread_setstatus:
 *
 *      Set the status of the specified thread.
 *      Called with (and returns with) no locks held.
 */
kern_return_t
thread_setstatus(
        thread_act_t            thr_act,
        int                     flavor,
        thread_state_t          tstate,
        mach_msg_type_number_t  count)
{
        kern_return_t           kr;
        thread_t                thread;

        thread = act_lock_thread(thr_act);
        assert(thread);
        assert(thread->top_act == thr_act);
        kr = act_machine_set_state(thr_act, flavor, tstate, count);
        act_unlock_thread(thr_act);
        return(kr);
}

/*
 *      thread_getstatus:
 *
 *      Get the status of the specified thread.
 */
kern_return_t
thread_getstatus(
        thread_act_t            thr_act,
        int                     flavor,
        thread_state_t          tstate,
        mach_msg_type_number_t  *count)
{
        kern_return_t           kr;
        thread_t                thread;

        thread = act_lock_thread(thr_act);
        assert(thread);
        assert(thread->top_act == thr_act);
        kr = act_machine_get_state(thr_act, flavor, tstate, count);
        act_unlock_thread(thr_act);
        return(kr);
}

/*
 * Kernel-internal thread_activation interfaces used outside this file:
 */

/*
 * act_init()   - Initialize activation handling code
 */
void
act_init()
{
        thr_act_zone = zinit(
                        sizeof(struct thread_activation),
                        ACT_MAX * sizeof(struct thread_activation), /* XXX */
                        ACT_CHUNK * sizeof(struct thread_activation),
                        "activations");
        act_machine_init();
}


/*
 * act_create   - Create a new activation in a specific task.
 */
kern_return_t
act_create(task_t task,
           thread_act_t *new_act)
{
        thread_act_t thr_act;
        int rc;
        vm_map_t map;

        thr_act = (thread_act_t)zalloc(thr_act_zone);
        if (thr_act == 0)
                return(KERN_RESOURCE_SHORTAGE);

#if     MACH_ASSERT
        if (watchacts & WA_ACT_LNK)
                printf("act_create(task=%x,thr_act@%x=%x)\n",
                        task, new_act, thr_act);
#endif  /* MACH_ASSERT */

        /* Start by zeroing everything; then init non-zero items only */
        bzero((char *)thr_act, sizeof(*thr_act));

#ifdef MACH_BSD
        {
                /*
                 * Take care of the uthread allocation
                 * do it early in order to make KERN_RESOURCE_SHORTAGE
                 * handling trivial
                 * uthread_alloc() will bzero the storage allocated.
                 */
                extern void *uthread_alloc(void);
                thr_act->uthread = uthread_alloc();
                if(thr_act->uthread == 0) {
                        /* Put the thr_act back on the thr_act zone */
                        zfree(thr_act_zone, (vm_offset_t)thr_act);
                        return(KERN_RESOURCE_SHORTAGE);
                }
        }
#endif  /* MACH_BSD */

        /*
         * Start with one reference for the caller and one for the
         * act being alive.
         */
        act_lock_init(thr_act);
        thr_act->ref_count = 2;

        /* Latch onto the task.  */
        thr_act->task = task;
        task_reference(task);

        /* Initialize sigbufp for High-Watermark buffer allocation */
        thr_act->r_sigbufp = (routine_descriptor_t) &thr_act->r_sigbuf;
        thr_act->r_sigbuf_size = sizeof(thr_act->r_sigbuf);

#if     THREAD_SWAPPER
        thr_act->swap_state = TH_SW_IN;
#if     MACH_ASSERT
        thr_act->kernel_stack_swapped_in = TRUE;
#endif  /* MACH_ASSERT */
#endif  /* THREAD_SWAPPER */

        /* special_handler will always be last on the returnhandlers list.  */
        thr_act->special_handler.next = 0;
        thr_act->special_handler.handler = special_handler;

#if     MACH_PROF
        thr_act->act_profiled = FALSE;
        thr_act->act_profiled_own = FALSE;
        thr_act->profil_buffer = NULLPROFDATA;
#endif

        /* Initialize the held_ulocks queue as empty */
        queue_init(&thr_act->held_ulocks);

        /* Inherit the profiling status of the parent task */
        act_prof_init(thr_act, task);

        ipc_thr_act_init(task, thr_act);
        act_machine_create(task, thr_act);

        /*
         * If thr_act created in kernel-loaded task, alter its saved
         * state to so indicate
         */
        if (task->kernel_loaded) {
                act_user_to_kernel(thr_act);
        }

        /* Cache the task's map and take a reference to it */
        map = task->map;
        thr_act->map = map;

        /* Inline vm_map_reference cause we don't want to increment res_count */
        mutex_lock(&map->s_lock);
#if     TASK_SWAPPER
        assert(map->res_count > 0);
        assert(map->ref_count >= map->res_count);
#endif  /* TASK_SWAPPER */
        map->ref_count++;
        mutex_unlock(&map->s_lock);

        *new_act = thr_act;
        return KERN_SUCCESS;
}

/*
 * act_free     - called when an thr_act's ref_count drops to zero.
 *
 * This can only happen after the activation has been reaped, and
 * all other references to it have gone away.  We can now release
 * the last critical resources, unlink the activation from the
 * task, and release the reference on the thread shuttle itself.
 *
 * Called with activation locked.
 */
#if     MACH_ASSERT
int     dangerous_bzero = 1;    /* paranoia & safety */
#endif

void
act_free(thread_act_t thr_act)
{
        task_t          task;
        thread_t        thr;
        vm_map_t        map;
        unsigned int    ref;

#if     MACH_ASSERT
        if (watchacts & WA_EXIT)
                printf("act_free(%x(%d)) thr=%x tsk=%x(%d) pport=%x%sactive\n",
                        thr_act, thr_act->ref_count, thr_act->thread,
                        thr_act->task,
                        thr_act->task ? thr_act->task->ref_count : 0,
                        thr_act->pool_port,
                        thr_act->active ? " " : " !");
#endif  /* MACH_ASSERT */


#if     THREAD_SWAPPER
        assert(thr_act->kernel_stack_swapped_in);
#endif  /* THREAD_SWAPPER */

        assert(!thr_act->active);
        assert(!thr_act->pool_port);

        task = thr_act->task;
        task_lock(task);

        if (thr = thr_act->thread) {
                time_value_t    user_time, system_time;

                thread_read_times(thr, &user_time, &system_time);
                time_value_add(&task->total_user_time, &user_time);
                time_value_add(&task->total_system_time, &system_time);
        
                /* Unlink the thr_act from the task's thr_act list,
                 * so it doesn't appear in calls to task_threads and such.
                 * The thr_act still keeps its ref on the task, however.
                 */
                queue_remove(&task->thr_acts, thr_act, thread_act_t, thr_acts);
                thr_act->thr_acts.next = NULL;
                task->thr_act_count--;

#if     THREAD_SWAPPER
                /*
                 * Thread is supposed to be unswappable by now...
                 */
                assert(thr_act->swap_state == TH_SW_UNSWAPPABLE ||
                       !thread_swap_unwire_stack);
#endif  /* THREAD_SWAPPER */

                task->res_act_count--;
                task_unlock(task);
                task_deallocate(task);
                thread_deallocate(thr);
                act_machine_destroy(thr_act);
        } else {
                /*
                 * Must have never really gotten started
                 * no unlinking from the task and no need
                 * to free the shuttle.
                 */
                task_unlock(task);
                task_deallocate(task);
        }

        sigbuf_dealloc(thr_act);
        act_prof_deallocate(thr_act);
        ipc_thr_act_terminate(thr_act);

        /*
         * Drop the cached map reference.
         * Inline version of vm_map_deallocate() because we
         * don't want to decrement the map's residence count here.
         */
        map = thr_act->map;
        mutex_lock(&map->s_lock);
#if     TASK_SWAPPER
        assert(map->res_count >= 0);
        assert(map->ref_count > map->res_count);
#endif  /* TASK_SWAPPER */
        ref = --map->ref_count;
        mutex_unlock(&map->s_lock);
        if (ref == 0)
                vm_map_destroy(map);

#ifdef MACH_BSD 
        {
                /*
                 * Free uthread BEFORE the bzero.
                 * Not doing so will result in a leak.
                 */
                extern void uthread_free(void *);
                void *ut = thr_act->uthread;
                thr_act->uthread = 0;
                uthread_free(ut);
        }
#endif  /* MACH_BSD */   

#if     MACH_ASSERT
        if (dangerous_bzero)    /* dangerous if we're still using it! */
                bzero((char *)thr_act, sizeof(*thr_act));
#endif  /* MACH_ASSERT */
        /* Put the thr_act back on the thr_act zone */
        zfree(thr_act_zone, (vm_offset_t)thr_act);
}


/*
 * act_attach   - Attach an thr_act to the top of a thread ("push the stack").
 *
 * The thread_shuttle must be either the current one or a brand-new one.
 * Assumes the thr_act is active but not in use, also, that if it is
 * attached to an thread_pool (i.e. the thread_pool pointer is nonzero),
 * the thr_act has already been taken off the thread_pool's list.
 *
 * Already locked: thr_act plus "appropriate" thread-related locks
 * (see act_lock_thread()).
 */
void 
act_attach(
        thread_act_t    thr_act,
        thread_t        thread,
        unsigned        init_alert_mask)
{
        thread_act_t    lower;

#if     MACH_ASSERT
        assert(thread == current_thread() || thread->top_act == THR_ACT_NULL);
        if (watchacts & WA_ACT_LNK)
                printf("act_attach(thr_act %x(%d) thread %x(%d) mask %d)\n",
                       thr_act, thr_act->ref_count, thread, thread->ref_count,
                       init_alert_mask);
#endif  /* MACH_ASSERT */

        /* 
         *      Chain the thr_act onto the thread's thr_act stack.  
         *      Set mask and auto-propagate alerts from below.
         */
        thr_act->ref_count++;
        thr_act->thread = thread;
        thr_act->higher = THR_ACT_NULL;  /*safety*/
        thr_act->alerts = 0;
        thr_act->alert_mask = init_alert_mask;
        lower = thr_act->lower = thread->top_act;

        if (lower != THR_ACT_NULL) {
                lower->higher = thr_act;
                thr_act->alerts = (lower->alerts & init_alert_mask);
        }

        thread->top_act = thr_act;
}

/*
 *      act_detach      
 *
 *      Remove the current thr_act from the top of the current thread, i.e.
 *      "pop the stack". Assumes already locked: thr_act plus "appropriate"
 *      thread-related locks (see act_lock_thread).
 */
void 
act_detach(
        thread_act_t    cur_act)
{
        thread_t        cur_thread = cur_act->thread;

#if     MACH_ASSERT
        if (watchacts & (WA_EXIT|WA_ACT_LNK))
                printf("act_detach: thr_act %x(%d), thrd %x(%d) task=%x(%d)\n",
                       cur_act, cur_act->ref_count,
                       cur_thread, cur_thread->ref_count,
                       cur_act->task,
                       cur_act->task ? cur_act->task->ref_count : 0);
#endif  /* MACH_ASSERT */

        /* Unlink the thr_act from the thread's thr_act stack */
        cur_thread->top_act = cur_act->lower;
        cur_act->thread = 0;
        cur_act->ref_count--;
        assert(cur_act->ref_count > 0);

        thread_pool_put_act(cur_act);

#if     MACH_ASSERT
        cur_act->lower = cur_act->higher = THR_ACT_NULL; 
        if (cur_thread->top_act)
                cur_thread->top_act->higher = THR_ACT_NULL;
#endif  /* MACH_ASSERT */

        return;
}


/*
 * Synchronize a thread operation with RPC.  Called with nothing
 * locked.   Returns with thr_act locked, plus one of four
 * combinations of other locks held:
 *      none - for new activation not yet associated with thread_pool
 *              or shuttle
 *      rpc_lock(thr_act->thread) only - for base activation (one
 *              without pool_port)
 *      ip_lock(thr_act->pool_port) only - for empty activation (one
 *              with no associated shuttle)
 *      both locks - for "active" activation (has shuttle, lives
 *              on thread_pool)
 * If thr_act has an associated shuttle, this function returns
 * its address.  Otherwise it returns zero.
 */
thread_t
act_lock_thread(
        thread_act_t thr_act)
{
        ipc_port_t pport;

        /*
         * Allow the shuttle cloning code (q.v., when it
         * exists :-}) to obtain ip_lock()'s while holding
         * an rpc_lock().
         */
        while (1) {
                act_lock(thr_act);
                pport = thr_act->pool_port;
                if (!pport || ip_lock_try(pport)) {
                        if (!thr_act->thread)
                                break;
                        if (rpc_lock_try(thr_act->thread))
                                break;
                        if (pport)
                                ip_unlock(pport);
                }
                act_unlock(thr_act);
                mutex_pause();
        }
        return (thr_act->thread);
}

/*
 * Unsynchronize with RPC (i.e., undo an act_lock_thread() call).
 * Called with thr_act locked, plus thread locks held that are
 * "correct" for thr_act's state.  Returns with nothing locked.
 */
void
act_unlock_thread(thread_act_t  thr_act)
{
        if (thr_act->thread)
                rpc_unlock(thr_act->thread);
        if (thr_act->pool_port)
                ip_unlock(thr_act->pool_port);
        act_unlock(thr_act);
}

/*
 * Synchronize with RPC given a pointer to a shuttle (instead of an
 * activation).  Called with nothing locked; returns with all
 * "appropriate" thread-related locks held (see act_lock_thread()).
 */
thread_act_t
thread_lock_act(
        thread_t        thread)
{
        thread_act_t    thr_act;

        while (1) {
                rpc_lock(thread);
                thr_act = thread->top_act;
                if (!thr_act)
                        break;
                if (!act_lock_try(thr_act)) {
                        rpc_unlock(thread);
                        mutex_pause();
                        continue;
                }
                if (thr_act->pool_port &&
                        !ip_lock_try(thr_act->pool_port)) {
                        rpc_unlock(thread);
                        act_unlock(thr_act);
                        mutex_pause();
                        continue;
                }
                break;
        }
        return (thr_act);
}

/*
 * Unsynchronize with RPC starting from a pointer to a shuttle.
 * Called with RPC-related locks held that are appropriate to
 * shuttle's state; any activation is also locked.
 */
void
thread_unlock_act(
        thread_t        thread)
{
        thread_act_t    thr_act;

        if (thr_act = thread->top_act) {
                if (thr_act->pool_port)
                        ip_unlock(thr_act->pool_port);
                act_unlock(thr_act);
        }
        rpc_unlock(thread);
}

/*
 * switch_act
 *
 * If a new activation is given, switch to it. If not,
 * switch to the lower activation (pop). Returns the old
 * activation. This is for RPC support.
 */
thread_act_t
switch_act( 
        thread_act_t act)
{
        thread_t        thread;
        thread_act_t    old, new;
        unsigned        cpu;
        spl_t           spl;


        disable_preemption();

        cpu = cpu_number();
        thread  = current_thread();

        /*
         *      Find the old and new activation for switch.
         */
        old = thread->top_act;

        if (act) {
                new = act;
                new->thread = thread;
        }
        else {
                new = old->lower;
        }

        assert(new != THR_ACT_NULL);
#if     THREAD_SWAPPER
        assert(new->swap_state != TH_SW_OUT &&
               new->swap_state != TH_SW_COMING_IN);
#endif  /* THREAD_SWAPPER */

        assert(cpu_data[cpu].active_thread == thread);
        active_kloaded[cpu] = (new->kernel_loaded) ? new : 0;

        /* This is where all the work happens */
        machine_switch_act(thread, old, new, cpu);

        /*
         *      Push or pop an activation on the chain.
         */
        if (act) {
                act_attach(new, thread, 0);
        }
        else {
                act_detach(old);
        }

        enable_preemption();

        return(old);
}

/*
 * install_special_handler
 *      Install the special returnhandler that handles suspension and
 *      termination, if it hasn't been installed already.
 *
 * Already locked: RPC-related locks for thr_act, but not
 * scheduling lock (thread_lock()) of the associated thread.
 */
void
install_special_handler(
        thread_act_t    thr_act)
{
        spl_t           spl;
        thread_t        thread = thr_act->thread;

#if     MACH_ASSERT
        if (watchacts & WA_ACT_HDLR)
            printf("act_%x: install_special_hdlr(%x)\n",current_act(),thr_act);
#endif  /* MACH_ASSERT */

        spl = splsched();
        if (thread)
                thread_lock(thread);
        install_special_handler_locked(thr_act);
        act_set_apc(thr_act);
        if (thread)
                thread_unlock(thread);
        splx(spl);
}

/*
 * install_special_handler_locked
 *      Do the work of installing the special_handler.
 *
 * Already locked: RPC-related locks for thr_act, plus the
 * scheduling lock (thread_lock()) of the associated thread.
 */
void
install_special_handler_locked(
        thread_act_t    thr_act)
{
        ReturnHandler   **rh;
        thread_t        thread = thr_act->thread;

        /* The work handler must always be the last ReturnHandler on the list,
           because it can do tricky things like detach the thr_act.  */
        for (rh = &thr_act->handlers; *rh; rh = &(*rh)->next)
                /* */ ;
        if (rh != &thr_act->special_handler.next) {
                *rh = &thr_act->special_handler;
        }
        if (thread && thr_act == thread->top_act) {
                /*
                 * Temporarily undepress, so target has
                 * a chance to do locking required to
                 * block itself in special_handler().
                 */
                if (thread->depress_priority >= 0) {
                        thread->priority = thread->depress_priority;

                        /*
                         * Use special value -2 to indicate need
                         * to redepress priority in special_handler
                         * as thread blocks
                         */
                        thread->depress_priority = -2;
                        compute_priority(thread, FALSE);
                }       
        }
        act_set_apc(thr_act);
}

/*
 * JMM -
 * These two routines will be enhanced over time to call the general handler registration
 * mechanism used by special handlers and alerts.  They are hack in for now to avoid
 * having to export the gory details of ASTs to the BSD code right now.
 */
extern thread_apc_handler_t bsd_ast;

kern_return_t
thread_apc_set(
        thread_act_t thr_act,
        thread_apc_handler_t apc)
{
        assert(apc == bsd_ast);
        thread_ast_set(thr_act, AST_BSD);
        if (thr_act == current_act())
                ast_propagate(thr_act->ast);
        return KERN_SUCCESS;
}

kern_return_t
thread_apc_clear(
        thread_act_t thr_act,
        thread_apc_handler_t apc)
{
        assert(apc == bsd_ast);
        thread_ast_clear(thr_act, AST_BSD);
        if (thr_act == current_act())
                ast_off(AST_BSD);
        return KERN_SUCCESS;
}

/*
 * act_set_thread_pool  - Assign an activation to a specific thread_pool.
 * Fails if the activation is already assigned to another pool.
 * If thread_pool == 0, we remove the thr_act from its thread_pool.
 *
 * Called the port containing thread_pool already locked.
 * Returns the same way.
 */
kern_return_t act_set_thread_pool(
        thread_act_t    thr_act,
        ipc_port_t      pool_port)
{
        thread_pool_t   thread_pool;

#if     MACH_ASSERT
        if (watchacts & WA_ACT_LNK)
                printf("act_set_thread_pool: %x(%d) -> %x\n",
                        thr_act, thr_act->ref_count, thread_pool);
#endif  /* MACH_ASSERT */

        if (pool_port == 0) {
                thread_act_t *lact;

                if (thr_act->pool_port == 0)
                        return KERN_SUCCESS;
                thread_pool = &thr_act->pool_port->ip_thread_pool;

                for (lact = &thread_pool->thr_acts; *lact;
                                        lact = &((*lact)->thread_pool_next)) {
                        if (thr_act == *lact) {
                                *lact = thr_act->thread_pool_next;
                                break;
                        }
                }
                act_lock(thr_act);
                thr_act->pool_port = 0;
                thr_act->thread_pool_next = 0;
                act_unlock(thr_act);
                act_deallocate(thr_act);
                return KERN_SUCCESS;
        }
        if (thr_act->pool_port != pool_port) {
                thread_pool = &pool_port->ip_thread_pool;
                if (thr_act->pool_port != 0) {
#if     MACH_ASSERT
                        if (watchacts & WA_ACT_LNK)
                            printf("act_set_thread_pool found %x!\n",
                                                        thr_act->pool_port);
#endif  /* MACH_ASSERT */
                        return(KERN_FAILURE);
                }
                act_lock(thr_act);
                thr_act->pool_port = pool_port;

                /* The pool gets a ref to the activation -- have
                 * to inline operation because thr_act is already
                 * locked.
                 */
                act_locked_act_reference(thr_act);

                /* If it is available,
                 * add it to the thread_pool's available-activation list.
                 */
                if ((thr_act->thread == 0) && (thr_act->suspend_count == 0)) {
                        thr_act->thread_pool_next = thread_pool->thr_acts;
                        pool_port->ip_thread_pool.thr_acts = thr_act;
                        if (thread_pool->waiting)
                                thread_pool_wakeup(thread_pool);
                }
                act_unlock(thr_act);
        }

        return KERN_SUCCESS;
}

/*
 * act_locked_act_set_thread_pool- Assign activation to a specific thread_pool.
 * Fails if the activation is already assigned to another pool.
 * If thread_pool == 0, we remove the thr_act from its thread_pool.
 *
 * Called the port containing thread_pool already locked.
 * Also called with the thread activation locked.
 * Returns the same way.
 *
 * This routine is the same as `act_set_thread_pool()' except that it does
 * not call `act_deallocate(),' which unconditionally tries to obtain the
 * thread activation lock.
 */
kern_return_t act_locked_act_set_thread_pool(
        thread_act_t    thr_act,
        ipc_port_t      pool_port)
{
        thread_pool_t   thread_pool;

#if     MACH_ASSERT
        if (watchacts & WA_ACT_LNK)
                printf("act_set_thread_pool: %x(%d) -> %x\n",
                        thr_act, thr_act->ref_count, thread_pool);
#endif  /* MACH_ASSERT */

        if (pool_port == 0) {
                thread_act_t *lact;

                if (thr_act->pool_port == 0)
                        return KERN_SUCCESS;
                thread_pool = &thr_act->pool_port->ip_thread_pool;

                for (lact = &thread_pool->thr_acts; *lact;
                                        lact = &((*lact)->thread_pool_next)) {
                        if (thr_act == *lact) {
                                *lact = thr_act->thread_pool_next;
                                break;
                        }
                }

                thr_act->pool_port = 0;
                thr_act->thread_pool_next = 0;
                act_locked_act_deallocate(thr_act);
                return KERN_SUCCESS;
        }
        if (thr_act->pool_port != pool_port) {
                thread_pool = &pool_port->ip_thread_pool;
                if (thr_act->pool_port != 0) {
#if     MACH_ASSERT
                        if (watchacts & WA_ACT_LNK)
                            printf("act_set_thread_pool found %x!\n",
                                                        thr_act->pool_port);
#endif  /* MACH_ASSERT */
                        return(KERN_FAILURE);
                }
                thr_act->pool_port = pool_port;

                /* The pool gets a ref to the activation -- have
                 * to inline operation because thr_act is already
                 * locked.
                 */
                act_locked_act_reference(thr_act);

                /* If it is available,
                 * add it to the thread_pool's available-activation list.
                 */
                if ((thr_act->thread == 0) && (thr_act->suspend_count == 0)) {
                        thr_act->thread_pool_next = thread_pool->thr_acts;
                        pool_port->ip_thread_pool.thr_acts = thr_act;
                        if (thread_pool->waiting)
                                thread_pool_wakeup(thread_pool);
                }
        }

        return KERN_SUCCESS;
}

/*
 * Activation control support routines internal to this file:
 */

/*
 * act_execute_returnhandlers() - does just what the name says
 *
 * This is called by system-dependent code when it detects that
 * thr_act->handlers is non-null while returning into user mode.
 * Activations linked onto an thread_pool always have null thr_act->handlers,
 * so RPC entry paths need not check it.
 */
void act_execute_returnhandlers(
        void)
{
        spl_t           s;
        thread_t        thread;
        thread_act_t    thr_act = current_act();

#if     MACH_ASSERT
        if (watchacts & WA_ACT_HDLR)
                printf("execute_rtn_hdlrs: thr_act=%x\n", thr_act);
#endif  /* MACH_ASSERT */

        s = splsched();
        act_clr_apc(thr_act);
        spllo();
        while (1) {
                ReturnHandler *rh;

                /* Grab the next returnhandler */
                thread = act_lock_thread(thr_act);
                (void)splsched();
                thread_lock(thread);
                rh = thr_act->handlers;
                if (!rh) {
                        thread_unlock(thread);
                        splx(s);
                        act_unlock_thread(thr_act);
                        return;
                }
                thr_act->handlers = rh->next;
                thread_unlock(thread);
                spllo();
                act_unlock_thread(thr_act);

#if     MACH_ASSERT
                if (watchacts & WA_ACT_HDLR)
                    printf( (rh == &thr_act->special_handler) ?
                        "\tspecial_handler\n" : "\thandler=%x\n",
                                    rh->handler);
#endif  /* MACH_ASSERT */

                /* Execute it */
                (*rh->handler)(rh, thr_act);
        }
}

/*
 * special_handler_continue
 *
 * Continuation routine for the special handler blocks.  It checks
 * to see whether there has been any new suspensions.  If so, it
 * installs the special handler again.  Otherwise, it checks to see
 * if the current depression needs to be re-instated (it may have
 * been temporarily removed in order to get to this point in a hurry).
 */
void
special_handler_continue(void)
{
        thread_act_t cur_act = current_act();
        thread_t thread = cur_act->thread;
        spl_t s;

        if (cur_act->suspend_count)
                install_special_handler(cur_act);
        else {
                s = splsched();
                thread_lock(thread);
                if (thread->depress_priority == -2) {
                        /*
                         * We were temporarily undepressed by
                         * install_special_handler; restore priority
                         * depression.
                         */
                        thread->depress_priority = thread->priority;
                        thread->priority = thread->sched_pri = DEPRESSPRI;
                }
                thread_unlock(thread);
                splx(s);
        }
        thread_exception_return();
}

/*
 * special_handler      - handles suspension, termination.  Called
 * with nothing locked.  Returns (if it returns) the same way.
 */
void
special_handler(
        ReturnHandler   *rh,
        thread_act_t    cur_act)
{
        spl_t           s;
        thread_t        lthread;
        thread_t        thread = act_lock_thread(cur_act);
        unsigned        alert_bits;
        exception_data_type_t
                        codes[EXCEPTION_CODE_MAX];
        kern_return_t   kr;
        kern_return_t   exc_kr;

        assert(thread != THREAD_NULL);
#if     MACH_ASSERT
        if (watchacts & WA_ACT_HDLR)
            printf("\t\tspecial_handler(thr_act=%x(%d))\n", cur_act,
                                (cur_act ? cur_act->ref_count : 0));
#endif  /* MACH_ASSERT */

        s = splsched();

        thread_lock(thread);
        thread->state &= ~TH_ABORT;     /* clear any aborts */
        thread_unlock(thread);
        splx(s);

        /*
         * If someone has killed this invocation,
         * invoke the return path with a terminated exception.
         */
        if (!cur_act->active) {
                act_unlock_thread(cur_act);
                act_machine_return(KERN_TERMINATED);
        }

#ifdef CALLOUT_RPC_MODEL
        /*
         * JMM - We don't intend to support this RPC model in Darwin.
         * We will support inheritance through chains of activations
         * on shuttles, but it will be universal and not just for RPC.
         * As such, each activation will always have a base shuttle.
         * Our RPC model will probably even support the notion of
         * alerts (thrown up the chain of activations to affect the
         * work done on our behalf), but the unlinking of the shuttles
         * will be completely difference because we will never have
         * to clone them.
         */

        /* strip server terminated bit */
        alert_bits = cur_act->alerts & (~SERVER_TERMINATED);

        /* clear server terminated bit */
        cur_act->alerts &= ~SERVER_TERMINATED;

        if ( alert_bits ) {
                /*
                 * currently necessary to coordinate with the exception 
                 * code -fdr
                 */
                act_unlock_thread(cur_act);

                /* upcall exception/alert port */
                codes[0] = alert_bits;

                /*
                 * Exception makes a lot of assumptions. If there is no
                 * exception handler or the exception reply is broken, the 
                 * thread will be terminated and exception will not return. If
                 * we decide we don't like that behavior, we need to check
                 * for the existence of an exception port before we call 
                 * exception.
                 */
                exc_kr = exception( EXC_RPC_ALERT, codes, 1 );

                /* clear the orphaned and time constraint indications */
                cur_act->alerts &= ~(ORPHANED | TIME_CONSTRAINT_UNSATISFIED);

                /* if this orphaned activation should be terminated... */
                if (exc_kr == KERN_RPC_TERMINATE_ORPHAN) {
                        /*
                         * ... terminate the activation
                         *
                         * This is done in two steps.  First, the activation is
                         * disabled (prepared for termination); second, the
                         * `special_handler()' is executed again -- this time
                         * to terminate the activation.
                         * (`act_disable_task_locked()' arranges for the
                         * additional execution of the `special_handler().')
                         */

#if     THREAD_SWAPPER
                        thread_swap_disable(cur_act);
#endif  /* THREAD_SWAPPER */

                        /* acquire appropriate locks */
                        task_lock(cur_act->task);
                        act_lock_thread(cur_act);

                        /* detach the activation from its task */
                        kr = act_disable_task_locked(cur_act);
                        assert( kr == KERN_SUCCESS );

                        /* release locks */
                        task_unlock(cur_act->task);
                }
                else {
                        /* acquire activation lock again (released below) */
                        act_lock_thread(cur_act);
                        s = splsched();
                        thread_lock(thread);
                        if (thread->depress_priority == -2) {
                                /*
                                 * We were temporarily undepressed by
                                 * install_special_handler; restore priority
                                 * depression.
                                 */
                                thread->depress_priority = thread->priority;
                                thread->priority = thread->sched_pri = DEPRESSPRI;
                        }
                        thread_unlock(thread);
                        splx(s);
                }
        }
#endif /* CALLOUT_RPC_MODEL */

        /*
         * If we're suspended, go to sleep and wait for someone to wake us up.
         */
        if (cur_act->suspend_count) {
                if( cur_act->handlers == NULL ) {
                        assert_wait((event_t)&cur_act->suspend_count,
                                    THREAD_ABORTSAFE);
                        act_unlock_thread(cur_act);
                        thread_block(special_handler_continue);
                        /* NOTREACHED */
                }
                special_handler_continue();
        }

        act_unlock_thread(cur_act);
}

/*
 * Try to nudge a thr_act into executing its returnhandler chain.
 * Ensures that the activation will execute its returnhandlers
 * before it next executes any of its user-level code.
 *
 * Called with thr_act's act_lock() and "appropriate" thread-related
 * locks held.  (See act_lock_thread().)  Returns same way.
 */
void
nudge(thread_act_t      thr_act)
{
#if     MACH_ASSERT
        if (watchacts & WA_ACT_HDLR)
            printf("\tact_%x: nudge(%x)\n", current_act(), thr_act);
#endif  /* MACH_ASSERT */

        /*
         * Don't need to do anything at all if this thr_act isn't the topmost.
         */
        if (thr_act->thread && thr_act->thread->top_act == thr_act) {
                /*
                 * If it's suspended, wake it up. 
                 * This should nudge it even on another CPU.
                 */
                thread_wakeup((event_t)&thr_act->suspend_count);
        }
}

/*
 * Update activation that belongs to a task created via kernel_task_create().
 */
void
act_user_to_kernel(
        thread_act_t    thr_act)
{
        pcb_user_to_kernel(thr_act);
        thr_act->kernel_loading = TRUE;
}

/*
 * Already locked: thr_act->task, RPC-related locks for thr_act
 *
 * Detach an activation from its task, and prepare it to terminate
 * itself.
 */
kern_return_t
act_disable_task_locked(
        thread_act_t    thr_act)
{
        thread_t        thread = thr_act->thread;
        task_t          task = thr_act->task;

#if     MACH_ASSERT
        if (watchacts & WA_EXIT) {
                printf("act_%x: act_disable_tl(thr_act=%x(%d))%sactive task=%x(%d)",
                               current_act(), thr_act, thr_act->ref_count,
                               (thr_act->active ? " " : " !"),
                               thr_act->task, thr_act->task? thr_act->task->ref_count : 0);
                if (thr_act->pool_port)
                        printf(", pool_port %x", thr_act->pool_port);
                printf("\n");
                (void) dump_act(thr_act);
        }
#endif  /* MACH_ASSERT */

        /* This will allow no more control ops on this thr_act. */
        thr_act->active = 0;
        ipc_thr_act_disable(thr_act);

        /* Clean-up any ulocks that are still owned by the thread
         * activation (acquired but not released or handed-off).
         */
        act_ulock_release_all(thr_act);

        /* When the special_handler gets executed,
         * it will see the terminated condition and exit
         * immediately.
         */
        install_special_handler(thr_act);


        /* If the target happens to be suspended,
         * give it a nudge so it can exit.
         */
        if (thr_act->suspend_count)
                nudge(thr_act);

        /* Drop the thr_act reference taken for being active.
         * (There is still at least one reference left:
         * the one we were passed.)
         * Inline the deallocate because thr_act is locked.
         */
        act_locked_act_deallocate(thr_act);

        return(KERN_SUCCESS);
}

/*
 * act_alert    - Register an alert from this activation.
 *
 * Each set bit is propagated upward from (but not including) this activation,
 * until the top of the chain is reached or the bit is masked.
 */
kern_return_t
act_alert(thread_act_t thr_act, unsigned alerts)
{
        thread_t thread = act_lock_thread(thr_act);

#if     MACH_ASSERT
        if (watchacts & WA_ACT_LNK)
                printf("act_alert %x: %x\n", thr_act, alerts);
#endif  /* MACH_ASSERT */

        if (thread) {
                thread_act_t act_up = thr_act;
                while ((alerts) && (act_up != thread->top_act)) {
                        act_up = act_up->higher;
                        alerts &= act_up->alert_mask;
                        act_up->alerts |= alerts;
                }
                /*
                 * XXXX If we reach the top, and it is blocked in glue
                 * code, do something to kick it.  XXXX
                 */
        }
        act_unlock_thread(thr_act);

        return KERN_SUCCESS;
}

kern_return_t act_alert_mask(thread_act_t thr_act, unsigned alert_mask)
{
        panic("act_alert_mask NOT YET IMPLEMENTED\n");
        return KERN_SUCCESS;
}

typedef struct GetSetState {
        struct ReturnHandler rh;
        int flavor;
        void *state;
        int *pcount;
        int result;
} GetSetState;

/* Local Forward decls */
kern_return_t get_set_state(
                        thread_act_t thr_act, int flavor,
                        thread_state_t state, int *pcount,
                        void (*handler)(ReturnHandler *rh, thread_act_t thr_act));
void get_state_handler(ReturnHandler *rh, thread_act_t thr_act);
void set_state_handler(ReturnHandler *rh, thread_act_t thr_act);

/*
 * get_set_state(thr_act ...)
 *
 * General code to install g/set_state handler.
 * Called with thr_act's act_lock() and "appropriate"
 * thread-related locks held.  (See act_lock_thread().)
 */
kern_return_t
get_set_state(thread_act_t thr_act, int flavor, thread_state_t state, int *pcount,
                        void (*handler)(ReturnHandler *rh, thread_act_t thr_act))
{
        GetSetState gss;
        spl_t s;

        /* Initialize a small parameter structure */
        gss.rh.handler = handler;
        gss.flavor = flavor;
        gss.state = state;
        gss.pcount = pcount;
        gss.result = KERN_ABORTED;      /* iff wait below is interrupted */

        /* Add it to the thr_act's return handler list */
        gss.rh.next = thr_act->handlers;
        thr_act->handlers = &gss.rh;

        s = splsched();
        act_set_apc(thr_act);
        splx(s);

#if     MACH_ASSERT
        if (watchacts & WA_ACT_HDLR) {
            printf("act_%x: get_set_state(thr_act=%x flv=%x state=%x ptr@%x=%x)",
                    current_act(), thr_act, flavor, state,
                    pcount, (pcount ? *pcount : 0));
            printf((handler == get_state_handler ? "get_state_hdlr\n" :
                    (handler == set_state_handler ? "set_state_hdlr\n" :
                        "hndler=%x\n")), handler); 
        }
#endif  /* MACH_ASSERT */

        assert(thr_act->thread);        /* Callers must ensure these */
        assert(thr_act != current_act());
        for (;;) {
                nudge(thr_act);
                /*
                 * Wait must be interruptible to avoid deadlock (e.g.) with
                 * task_suspend() when caller and target of get_set_state()
                 * are in same task.
                 */
                assert_wait((event_t)&gss, THREAD_ABORTSAFE);
                act_unlock_thread(thr_act);
                thread_block((void (*)(void))0);
                if (gss.result != KERN_ABORTED)
                        break;
                if (current_act()->handlers)
                        act_execute_returnhandlers();
                act_lock_thread(thr_act);
        }

#if     MACH_ASSERT
        if (watchacts & WA_ACT_HDLR)
            printf("act_%x: get_set_state returns %x\n",
                            current_act(), gss.result);
#endif  /* MACH_ASSERT */

        return gss.result;
}

void
set_state_handler(ReturnHandler *rh, thread_act_t thr_act)
{
        GetSetState *gss = (GetSetState*)rh;

#if     MACH_ASSERT
        if (watchacts & WA_ACT_HDLR)
                printf("act_%x: set_state_handler(rh=%x,thr_act=%x)\n",
                        current_act(), rh, thr_act);
#endif  /* MACH_ASSERT */

        gss->result = act_machine_set_state(thr_act, gss->flavor,
                                                gss->state, *gss->pcount);
        thread_wakeup((event_t)gss);
}

void
get_state_handler(ReturnHandler *rh, thread_act_t thr_act)
{
        GetSetState *gss = (GetSetState*)rh;

#if     MACH_ASSERT
        if (watchacts & WA_ACT_HDLR)
                printf("act_%x: get_state_handler(rh=%x,thr_act=%x)\n",
                        current_act(), rh, thr_act);
#endif  /* MACH_ASSERT */

        gss->result = act_machine_get_state(thr_act, gss->flavor,
                        gss->state, 
                        (mach_msg_type_number_t *) gss->pcount);
        thread_wakeup((event_t)gss);
}

kern_return_t
act_get_state_locked(thread_act_t thr_act, int flavor, thread_state_t state,
                                        mach_msg_type_number_t *pcount)
{
#if     MACH_ASSERT
    if (watchacts & WA_ACT_HDLR)
        printf("act_%x: act_get_state_L(thr_act=%x,flav=%x,st=%x,pcnt@%x=%x)\n",
                current_act(), thr_act, flavor, state, pcount,
                (pcount? *pcount : 0));
#endif  /* MACH_ASSERT */

    return(get_set_state(thr_act, flavor, state, (int*)pcount, get_state_handler));
}

kern_return_t
act_set_state_locked(thread_act_t thr_act, int flavor, thread_state_t state,
                                        mach_msg_type_number_t count)
{
#if     MACH_ASSERT
    if (watchacts & WA_ACT_HDLR)
        printf("act_%x: act_set_state_L(thr_act=%x,flav=%x,st=%x,pcnt@%x=%x)\n",
                current_act(), thr_act, flavor, state, count, count);
#endif  /* MACH_ASSERT */

    return(get_set_state(thr_act, flavor, state, (int*)&count, set_state_handler));
}

kern_return_t
act_set_state(thread_act_t thr_act, int flavor, thread_state_t state,
                                        mach_msg_type_number_t count)
{
    if (thr_act == THR_ACT_NULL || thr_act == current_act())
            return(KERN_INVALID_ARGUMENT);

    act_lock_thread(thr_act);
    return(act_set_state_locked(thr_act, flavor, state, count));
    
}

kern_return_t
act_get_state(thread_act_t thr_act, int flavor, thread_state_t state,
                                        mach_msg_type_number_t *pcount)
{
    if (thr_act == THR_ACT_NULL || thr_act == current_act())
            return(KERN_INVALID_ARGUMENT);

    act_lock_thread(thr_act);
    return(act_get_state_locked(thr_act, flavor, state, pcount));
}

/*
 * These two should be called at splsched()
 * Set/clear indicator to run APC (layered on ASTs)
 */
void
act_set_apc(thread_act_t thr_act)
{
        thread_ast_set(thr_act, AST_APC);
        if (thr_act == current_act()) {
                mp_disable_preemption();
                ast_propagate(thr_act->ast);
                mp_enable_preemption();
        }
}

void
act_clr_apc(thread_act_t thr_act)
{
        thread_ast_clear(thr_act, AST_APC);
}

void
act_ulock_release_all(thread_act_t thr_act)
{
        ulock_t ulock;

        while (!queue_empty(&thr_act->held_ulocks)) {
                ulock = (ulock_t) queue_first(&thr_act->held_ulocks);
                (void) lock_make_unstable(ulock, thr_act);
                (void) lock_release_internal(ulock, thr_act);
        }
}

/*
 * Provide routines (for export to other components) of things that
 * are implemented as macros insternally.
 */
#undef current_act
thread_act_t
current_act(void)
{
        return(current_act_fast());
}

thread_act_t
thread_self(void)
{
        thread_act_t self = current_act_fast();

        act_reference(self);
        return self;
}

thread_act_t
mach_thread_self(void)
{
        thread_act_t self = current_act_fast();

        act_reference(self);
        return self;
}

#undef act_reference
void
act_reference(
        thread_act_t thr_act)
{
        act_reference_fast(thr_act);
}

#undef act_deallocate
void
act_deallocate(
        thread_act_t thr_act) 
{
        act_deallocate_fast(thr_act);
}