xnu-517.3.7.tar.gz

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

/*
 * Copyright (c) 2000 Apple Computer, Inc. All rights reserved.
 *
 * @APPLE_LICENSE_HEADER_START@
 * 
 * Copyright (c) 1999-2003 Apple Computer, Inc.  All Rights Reserved.
 * 
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this
 * file.
 * 
 * The 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, QUIET ENJOYMENT 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/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/mk_sp.h> /*** ??? fix so this can be removed ***/
#include <kern/processor.h>
#include <mach_prof.h>
#include <mach/rpc.h>

/*
 * 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, boolean_t);
void            special_handler(ReturnHandler *, 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_astbsd(thread_act_t);
void            act_set_apc(thread_act_t);
void            act_ulock_release_all(thread_act_t thr_act);

void            install_special_handler_locked(thread_act_t);

static void             act_disable(thread_act_t);

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


/*
 * Internal routine to terminate a thread.
 * Sometimes called with task already locked.
 */
kern_return_t
thread_terminate_internal(
        register thread_act_t   act)
{
        kern_return_t   result;
        thread_t                thread;

        thread = act_lock_thread(act);

        if (!act->active) {
                act_unlock_thread(act);
                return (KERN_TERMINATED);
        }

        act_disable(act);
        result = act_abort(act, FALSE);

        /* 
         * Make sure this thread enters the kernel
         * Must unlock the act, but leave the shuttle
         * captured in this act.
         */
        if (thread != current_thread()) {
                act_unlock(act);

                if (thread_stop(thread))
                        thread_unstop(thread);
                else
                        result = KERN_ABORTED;

                act_lock(act);
        }

        clear_wait(thread, act->started? THREAD_INTERRUPTED: THREAD_AWAKENED);
        act_unlock_thread(act);

        return (result);
}

/*
 * Terminate a thread.
 */
kern_return_t
thread_terminate(
        register thread_act_t   act)
{
        kern_return_t   result;

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

        if (    act->task == kernel_task        &&
                        act != current_act()                    )
                return (KERN_FAILURE);

        result = thread_terminate_internal(act);

        /*
         * 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 (act->task == kernel_task) {
                ml_set_interrupts_enabled(FALSE);
                assert(act == current_act());
                ast_taken(AST_APC, TRUE);
                panic("thread_terminate");
        }

        return (result);
}

/*
 * Suspend execution of the specified thread.
 * This is a recursive-style suspension of the thread, a count of
 * suspends is maintained.
 *
 * Called with act_lock held.
 */
void
thread_hold(
        register thread_act_t   act)
{
        thread_t        thread = act->thread;

        if (act->suspend_count++ == 0) {
                install_special_handler(act);
                if (    act->started                            &&
                                thread != THREAD_NULL           &&
                                thread->top_act == act          )
                        thread_wakeup_one(&act->suspend_count);
        }
}

/*
 * Decrement internal suspension count for thr_act, setting thread
 * runnable when count falls to zero.
 *
 * Called with act_lock held.
 */
void
thread_release(
        register thread_act_t   act)
{
        thread_t        thread = act->thread;

        if (    act->suspend_count > 0          &&
                        --act->suspend_count == 0       &&
                        thread != THREAD_NULL           &&
                        thread->top_act == act          ) {
                if (!act->started) {
                        clear_wait(thread, THREAD_AWAKENED);
                        act->started = TRUE;
                }
                else
                        thread_wakeup_one(&act->suspend_count);
        }
}

kern_return_t
thread_suspend(
        register thread_act_t   act)
{
        thread_t        thread;

        if (act == THR_ACT_NULL || act->task == kernel_task)
                return (KERN_INVALID_ARGUMENT);

        thread = act_lock_thread(act);

        if (!act->active) {
                act_unlock_thread(act);
                return (KERN_TERMINATED);
        }

        if (    act->user_stop_count++ == 0             &&
                        act->suspend_count++ == 0               ) {
                install_special_handler(act);
                if (    thread != current_thread()              &&
                                thread != THREAD_NULL                   &&
                                thread->top_act == act                  ) {
                        assert(act->started);
                        thread_wakeup_one(&act->suspend_count);
                        act_unlock_thread(act);

                        thread_wait(thread);
                }
                else
                        act_unlock_thread(act);
        }
        else
                act_unlock_thread(act);

        return (KERN_SUCCESS);
}

kern_return_t
thread_resume(
        register thread_act_t   act)
{
        kern_return_t   result = KERN_SUCCESS;
        thread_t                thread;

        if (act == THR_ACT_NULL || act->task == kernel_task)
                return (KERN_INVALID_ARGUMENT);

        thread = act_lock_thread(act);

        if (act->active) {
                if (act->user_stop_count > 0) {
                        if (    --act->user_stop_count == 0             &&
                                        --act->suspend_count == 0               &&
                                        thread != THREAD_NULL                   &&
                                        thread->top_act == act                  ) {
                                if (!act->started) {
                                        clear_wait(thread, THREAD_AWAKENED);
                                        act->started = TRUE;
                                }
                                else
                                        thread_wakeup_one(&act->suspend_count);
                        }
                }
                else
                        result = KERN_FAILURE;
        }
        else
                result = KERN_TERMINATED;

        act_unlock_thread(act);

        return (result);
}

/*
 *      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;

    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);
    }

    result = _mk_sp_thread_depress_abort(thread, FALSE);

    act_unlock_thread(thr_act);

        return (result);
}


/*
 * Indicate that the activation should run its
 * special handler to detect the condition.
 *
 * Called with act_lock held.
 */
kern_return_t
act_abort(
        thread_act_t    act, 
        boolean_t       chain_break )
{
        thread_t        thread = act->thread;
        spl_t           s = splsched();

        assert(thread->top_act == act);

        thread_lock(thread);
        if (!(thread->state & TH_ABORT)) {
                thread->state |= TH_ABORT;
                install_special_handler_locked(act);
        } else {
                thread->state &= ~TH_ABORT_SAFELY;
        }
        thread_unlock(thread);
        splx(s);

        return (KERN_SUCCESS);
}
        
kern_return_t
thread_abort(
        register thread_act_t   act)
{
        kern_return_t   result;
        thread_t                thread;

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

        thread = act_lock_thread(act);

        if (!act->active) {
                act_unlock_thread(act);
                return (KERN_TERMINATED);
        }

        result = act_abort(act, FALSE);
        clear_wait(thread, THREAD_INTERRUPTED);
        act_unlock_thread(act);

        return (result);
}

kern_return_t
thread_abort_safely(
        thread_act_t    act)
{
        thread_t                thread;
        kern_return_t   ret;
        spl_t                   s;

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

        thread = act_lock_thread(act);

        if (!act->active) {
                act_unlock_thread(act);
                return (KERN_TERMINATED);
        }

        s = splsched();
        thread_lock(thread);
        if (!thread->at_safe_point ||
                clear_wait_internal(thread, THREAD_INTERRUPTED) != KERN_SUCCESS) {
                if (!(thread->state & TH_ABORT)) {
                        thread->state |= (TH_ABORT|TH_ABORT_SAFELY);
                        install_special_handler_locked(act);
                }
        }
        thread_unlock(thread);
        splx(s);
                
        act_unlock_thread(act);

        return (KERN_SUCCESS);
}

/*** 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 (!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 (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   act,
        int                                             flavor,
        thread_state_t                  state,                  /* pointer to OUT array */
        mach_msg_type_number_t  *state_count)   /*IN/OUT*/
{
        kern_return_t           result = KERN_SUCCESS;
        thread_t                        thread;

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

        thread = act_lock_thread(act);

        if (!act->active) {
                act_unlock_thread(act);
                return (KERN_TERMINATED);
        }

        thread_hold(act);

        for (;;) {
                thread_t                        thread1;

                if (    thread == THREAD_NULL           ||
                                thread->top_act != act          )
                        break;
                act_unlock_thread(act);

                if (!thread_stop(thread)) {
                        result = KERN_ABORTED;
                        (void)act_lock_thread(act);
                        thread = THREAD_NULL;
                        break;
                }
                        
                thread1 = act_lock_thread(act);
                if (thread1 == thread)
                        break;

                thread_unstop(thread);
                thread = thread1;
        }

        if (result == KERN_SUCCESS)
                result = machine_thread_get_state(act, flavor, state, state_count);

        if (    thread != THREAD_NULL           &&
                        thread->top_act == act          )
                thread_unstop(thread);

        thread_release(act);
        act_unlock_thread(act);

        return (result);
}

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

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

        thread = act_lock_thread(act);

        if (!act->active) {
                act_unlock_thread(act);
                return (KERN_TERMINATED);
        }

        thread_hold(act);

        for (;;) {
                thread_t                        thread1;

                if (    thread == THREAD_NULL           ||
                                thread->top_act != act          )
                        break;
                act_unlock_thread(act);

                if (!thread_stop(thread)) {
                        result = KERN_ABORTED;
                        (void)act_lock_thread(act);
                        thread = THREAD_NULL;
                        break;
                }

                thread1 = act_lock_thread(act);
                if (thread1 == thread)
                        break;

                thread_unstop(thread);
                thread = thread1;
        }

        if (result == KERN_SUCCESS)
                result = machine_thread_set_state(act, flavor, state, state_count);

        if (    thread != THREAD_NULL           &&
                        thread->top_act == act          )
            thread_unstop(thread);

        thread_release(act);
        act_unlock_thread(act);

        return (result);
}

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

kern_return_t
thread_dup(
        register thread_act_t   target)
{
        kern_return_t           result = KERN_SUCCESS;
        thread_act_t            self = current_act();
        thread_t                        thread;

        if (target == THR_ACT_NULL || target == self)
                return (KERN_INVALID_ARGUMENT);

        thread = act_lock_thread(target);

        if (!target->active) {
                act_unlock_thread(target);
                return (KERN_TERMINATED);
        }

        thread_hold(target);

        for (;;) {
                thread_t                        thread1;

                if (    thread == THREAD_NULL           ||
                                thread->top_act != target       )
                        break;
                act_unlock_thread(target);

                if (!thread_stop(thread)) {
                        result = KERN_ABORTED;
                        (void)act_lock_thread(target);
                        thread = THREAD_NULL;
                        break;
                }

                thread1 = act_lock_thread(target);
                if (thread1 == thread)
                        break;

                thread_unstop(thread);
                thread = thread1;
        }

        if (result == KERN_SUCCESS)
                result = machine_thread_dup(self, target);

        if (    thread != THREAD_NULL           &&
                        thread->top_act == target       )
            thread_unstop(thread);

        thread_release(target);
        act_unlock_thread(target);

        return (result);
}


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

        thread = act_lock_thread(act);

        if (    act != current_act()                    &&
                        (act->suspend_count == 0        ||
                         thread == THREAD_NULL          ||
                         (thread->state & TH_RUN)       ||
                         thread->top_act != act)                )
                result = KERN_FAILURE;

        if (result == KERN_SUCCESS)
                result = machine_thread_set_state(act, flavor, tstate, count);

        act_unlock_thread(act);

        return (result);
}

/*
 *      thread_getstatus:
 *
 *      Get the status of the specified thread.
 */
kern_return_t
thread_getstatus(
        register thread_act_t   act,
        int                                             flavor,
        thread_state_t                  tstate,
        mach_msg_type_number_t  *count)
{
        kern_return_t           result = KERN_SUCCESS;
        thread_t                        thread;

        thread = act_lock_thread(act);

        if (    act != current_act()                    &&
                        (act->suspend_count == 0        ||
                         thread == THREAD_NULL          ||
                         (thread->state & TH_RUN)       ||
                         thread->top_act != act)                )
                result = KERN_FAILURE;

        if (result == KERN_SUCCESS)
                result = machine_thread_get_state(act, flavor, tstate, count);

        act_unlock_thread(act);

        return (result);
}

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

void
act_reference(
        thread_act_t    act)
{
        if (act == NULL)
                return;

        act_lock(act);
        act_reference_locked(act);
        act_unlock(act);
}

void
act_deallocate(
        thread_act_t    act)
{
        task_t          task;
        thread_t        thread;
        void            *task_proc;

        if (act == NULL)
                return;

        act_lock(act);

        if (--act->act_ref_count > 0) {
                act_unlock(act);
                return;
        }

        assert(!act->active);

        thread = act->thread;
        assert(thread != NULL);

        thread->top_act = NULL;

        act_unlock(act);

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

        task_proc = task->bsd_info;

        {
                time_value_t    user_time, system_time;

                thread_read_times(thread, &user_time, &system_time);
                time_value_add(&task->total_user_time, &user_time);
                time_value_add(&task->total_system_time, &system_time);
        
                queue_remove(&task->threads, act, thread_act_t, task_threads);
                act->task_threads.next = NULL;
                task->thread_count--;
                task->res_thread_count--;
        }

        task_unlock(task);

        act_prof_deallocate(act);
        ipc_thr_act_terminate(act);

#ifdef MACH_BSD 
        {
                extern void uthread_free(task_t, void *, void *);
                void *ut = act->uthread;

                act->uthread = NULL;
                uthread_free(task, ut, task_proc);
        }
#endif  /* MACH_BSD */   

        task_deallocate(task);

        thread_deallocate(thread);
}


/*
 * 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.
 *
 * Already locked: thr_act plus "appropriate" thread-related locks
 * (see act_lock_thread()).
 */
void 
act_attach(
        thread_act_t            act,
        thread_t                        thread)
{
        thread_act_t    lower;

        /* 
         *      Chain the act onto the thread's act stack.  
         */
        act->act_ref_count++;
        act->thread = thread;
        act->higher = THR_ACT_NULL;
        lower = act->lower = thread->top_act;
        if (lower != THR_ACT_NULL)
                lower->higher = act;

        thread->top_act = 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;

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

#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 migration.
 * Called with nothing locked.
 * Returns with thr_act locked.
 */
thread_t
act_lock_thread(
        thread_act_t thr_act)
{

        /*
         * JMM - We have moved away from explicit RPC locks
         * and towards a generic migration approach.  The wait
         * queue lock will be the point of synchronization for
         * the shuttle linkage when this is rolled out.  Until
         * then, just lock the act.
         */
        act_lock(thr_act);
        return (thr_act->thread);
}

/*
 * Unsynchronize with migration (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)
{
        act_unlock(thr_act);
}

/*
 * Synchronize with migration 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) {
                thr_act = thread->top_act;
                if (!thr_act)
                        break;
                if (!act_lock_try(thr_act)) {
                        mutex_pause();
                        continue;
                }
                break;
        }
        return (thr_act);
}

/*
 * Unsynchronize with an activation starting from a pointer to
 * a shuttle.
 */
void
thread_unlock_act(
        thread_t        thread)
{
        thread_act_t    thr_act;

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

/*
 * 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 migration support.
 */
thread_act_t
switch_act( 
        thread_act_t act)
{
        thread_act_t    old, new;
        thread_t                thread;

        disable_preemption();

        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);
        assert(current_processor()->active_thread == thread);

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

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

        spl = splsched();
        thread_lock(thread);
        install_special_handler_locked(thr_act);
        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                            act)
{
        thread_t                thread = act->thread;
        ReturnHandler   **rh;

        /* 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 = &act->handlers; *rh; rh = &(*rh)->next)
                continue;
        if (rh != &act->special_handler.next)
                *rh = &act->special_handler;

        if (act == thread->top_act) {
                /*
                 * Temporarily undepress, so target has
                 * a chance to do locking required to
                 * block itself in special_handler().
                 */
                if (thread->sched_mode & TH_MODE_ISDEPRESSED)
                        compute_priority(thread, TRUE);
        }

        thread_ast_set(act, AST_APC);
        if (act == current_act())
                ast_propagate(act->ast);
        else {
                processor_t             processor = thread->last_processor;

                if (    processor != PROCESSOR_NULL                                     &&
                                processor->state == PROCESSOR_RUNNING           &&
                                processor->active_thread == thread                      )
                        cause_ast_check(processor);
        }
}

kern_return_t
thread_apc_set(
        thread_act_t                    act,
        thread_apc_handler_t    apc)
{
        extern thread_apc_handler_t     bsd_ast;

        assert(apc == bsd_ast);
        return (KERN_FAILURE);
}

kern_return_t
thread_apc_clear(
        thread_act_t                    act,
        thread_apc_handler_t    apc)
{
        extern thread_apc_handler_t     bsd_ast;

        assert(apc == bsd_ast);
        return (KERN_FAILURE);
}

/*
 * 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.
 */
void
act_execute_returnhandlers(void)
{
        thread_act_t    act = current_act();

        thread_ast_clear(act, AST_APC);
        spllo();

        for (;;) {
                ReturnHandler   *rh;
                thread_t                thread = act_lock_thread(act);

                (void)splsched();
                thread_lock(thread);
                rh = act->handlers;
                if (!rh) {
                        thread_unlock(thread);
                        spllo();
                        act_unlock_thread(act);
                        return;
                }
                act->handlers = rh->next;
                thread_unlock(thread);
                spllo();
                act_unlock_thread(act);

                /* Execute it */
                (*rh->handler)(rh, 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            self = current_act();

        if (self->suspend_count > 0)
                install_special_handler(self);
        else {
                thread_t                thread = self->thread;
                spl_t                   s = splsched();

                thread_lock(thread);
                if (thread->sched_mode & TH_MODE_ISDEPRESSED) {
                        processor_t             myprocessor = thread->last_processor;

                        thread->sched_pri = DEPRESSPRI;
                        myprocessor->current_pri = thread->sched_pri;
                        thread->sched_mode &= ~TH_MODE_PREEMPT;
                }
                thread_unlock(thread);
                splx(s);
        }

        thread_exception_return();
        /*NOTREACHED*/
}

/*
 * special_handler      - handles suspension, termination.  Called
 * with nothing locked.  Returns (if it returns) the same way.
 */
void
special_handler(
        ReturnHandler   *rh,
        thread_act_t    self)
{
        thread_t                thread = act_lock_thread(self);
        spl_t                   s;

        assert(thread != THREAD_NULL);

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

        if (!self->active) {
                act_unlock_thread(self);
                thread_terminate_self();
                /*NOTREACHED*/
        }

        /*
         * If we're suspended, go to sleep and wait for someone to wake us up.
         */
        if (self->suspend_count > 0) {
                if (self->handlers == NULL) {
                        assert_wait(&self->suspend_count, THREAD_ABORTSAFE);
                        act_unlock_thread(self);
                        thread_block(special_handler_continue);
                        /*NOTREACHED*/
                }

                act_unlock_thread(self);

                special_handler_continue();
                /*NOTREACHED*/
        }

        act_unlock_thread(self);
}

/*
 * Already locked: activation (shuttle frozen within)
 *
 * Mark an activation inactive, and prepare it to terminate
 * itself.
 */
static void
act_disable(
        thread_act_t    thr_act)
{
        thr_act->active = 0;

        /* 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_deallocate_locked(thr_act);
}

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            act,
        int                                     flavor,
        thread_state_t          state,
        int                                     *pcount,
        void                            (*handler)(
                                                        ReturnHandler   *rh,
                                                        thread_act_t     act))
{
        GetSetState                     gss;

        /* 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 = act->handlers;
        act->handlers = &gss.rh;

        act_set_apc(act);

        assert(act->thread);
        assert(act != current_act());

        for (;;) {
                wait_result_t           result;

                if (    act->started                                    &&
                                act->thread->top_act == act             )
                                thread_wakeup_one(&act->suspend_count);

                /*
                 * Wait must be interruptible to avoid deadlock (e.g.) with
                 * task_suspend() when caller and target of get_set_state()
                 * are in same task.
                 */
                result = assert_wait(&gss, THREAD_ABORTSAFE);
                act_unlock_thread(act);

                if (result == THREAD_WAITING)
                        result = thread_block(THREAD_CONTINUE_NULL);

                assert(result != THREAD_WAITING);

                if (gss.result != KERN_ABORTED) {
                        assert(result != THREAD_INTERRUPTED);
                        break;
                }

                /* JMM - What about other aborts (like BSD signals)? */
                if (current_act()->handlers)
                        act_execute_returnhandlers();

                act_lock_thread(act);
        }

        return (gss.result);
}

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

        gss->result = machine_thread_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;

        gss->result = machine_thread_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)
{
    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)
{
    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));
}

void
act_set_astbsd(
        thread_act_t    act)
{
        spl_t                   s = splsched();
        
        if (act == current_act()) {
                thread_ast_set(act, AST_BSD);
                ast_propagate(act->ast);
        }
        else {
                thread_t                thread = act->thread;
                processor_t             processor;

                thread_lock(thread);
                thread_ast_set(act, AST_BSD);
                processor = thread->last_processor;
                if (    processor != PROCESSOR_NULL                                     &&
                                processor->state == PROCESSOR_RUNNING           &&
                                processor->active_thread == thread                      )
                        cause_ast_check(processor);
                thread_unlock(thread);
        }
        
        splx(s);
}

void
act_set_apc(
        thread_act_t    act)
{
        spl_t                   s = splsched();
        
        if (act == current_act()) {
                thread_ast_set(act, AST_APC);
                ast_propagate(act->ast);
        }
        else {
                thread_t                thread = act->thread;
                processor_t             processor;

                thread_lock(thread);
                thread_ast_set(act, AST_APC);
                processor = thread->last_processor;
                if (    processor != PROCESSOR_NULL                                     &&
                                processor->state == PROCESSOR_RUNNING           &&
                                processor->active_thread == thread                      )
                        cause_ast_check(processor);
                thread_unlock(thread);
        }
        
        splx(s);
}

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.
 */
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;
}