libkern/os/refcnt.h

   1 /*
   2  * Copyright (c) 2017 Apple Inc. All rights reserved.
   3  *
   4  * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
   5  *
   6  * This file contains Original Code and/or Modifications of Original Code
   7  * as defined in and that are subject to the Apple Public Source License
   8  * Version 2.0 (the 'License'). You may not use this file except in
   9  * compliance with the License. The rights granted to you under the License
  10  * may not be used to create, or enable the creation or redistribution of,
  11  * unlawful or unlicensed copies of an Apple operating system, or to
  12  * circumvent, violate, or enable the circumvention or violation of, any
  13  * terms of an Apple operating system software license agreement.
  14  *
  15  * Please obtain a copy of the License at
  16  * http://www.opensource.apple.com/apsl/ and read it before using this file.
  17  *
  18  * The Original Code and all software distributed under the License are
  19  * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
  20  * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
  21  * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
  22  * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
  23  * Please see the License for the specific language governing rights and
  24  * limitations under the License.
  25  *
  26  * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
  27  */
  28
  29 #ifndef _OS_REFCNT_H_
  30 #define _OS_REFCNT_H_
  31
  32 /*
  33  * os_refcnt reference counting API
  34  *
  35  * Two flavors are provided: atomic and locked. Atomic internally uses C11 atomic
  36  * operations and requires no external synchronization, whereas the locked flavor
  37  * assumes the refcnt object is locked by the caller. It is NOT safe to
  38  * mix-and-match locked and atomic calls.
  39  *
  40  * 'refgrp's are a way to (hierarchically) group like refcount objects for
  41  * debugging purposes. The group keeps track of the total number and aggregate
  42  * reference count of member refcounts, and the "rlog=" boot-arg is used to enable
  43  * refcount logging by group name. Named groups can be created explicitly with
  44  * os_refgrp_decl(), or implicitly by passing NULL for the refgrp when
  45  * initializing a refcnt object. In the latter case, the group name is the same as
  46  * the function enclosing the init call. Groups are only available on DEV or DEBUG
  47  * builds, and are otherwise compiled out.
  48  */
  49
  50 #include <stdatomic.h>
  51 #include <stdbool.h>
  52 #include <os/base.h>
  53
  54 struct os_refcnt;
  55 struct os_refgrp;
  56 typedef struct os_refcnt os_refcnt_t;
  57
  58 /* type of the internal counter */
  59 typedef uint32_t os_ref_count_t;
  60 typedef _Atomic(os_ref_count_t) os_ref_atomic_t;
  61
  62 /*
  63  * OS_REF_INITIALIZER
  64  * OS_REF_ATOMIC_INITIALIZER
  65  *
  66  * Static initializers that create refcnt objects with safe initial values for use
  67  * between declaration and initialization (os_ref*_init()). Equivalent to zeroing.
  68  */
  69
  70 #ifndef KERNEL
  71 # include <stdlib.h>
  72 # include <stdio.h>
  73 # ifndef __improbable
  74 #  define __improbable(x) x
  75 # endif
  76 # ifndef panic
  77 #  define panic(x, ...) do { fprintf(stderr, x, __VA_ARGS__); abort(); } while (0)
  78 # endif
  79 #endif
  80
  81 #ifndef OS_REFCNT_DEBUG
  82 # if DEVELOPMENT || DEBUG
  83 #  define OS_REFCNT_DEBUG 1
  84 # else
  85 #  define OS_REFCNT_DEBUG 0
  86 # endif
  87 #endif
  88
  89 #if __has_attribute(diagnose_if)
  90 # define os_error_if(cond, msg) __attribute__((diagnose_if((cond), (msg), "error")))
  91 #else
  92 # define os_error_if(...)
  93 #endif
  94
  95 __BEGIN_DECLS
  96
  97 /*
  98  * os_ref_init: initialize an os_refcnt with a count of 1
  99  * os_ref_init_count: initialize an os_refcnt with a specific count >= 1
 100  */
 101 #define os_ref_init(rc, grp) os_ref_init_count((rc), (grp), 1)
 102 static void os_ref_init_count(struct os_refcnt *, struct os_refgrp *, os_ref_count_t count)
 103 os_error_if(count == 0, "Reference count must be non-zero initialized");
 104
 105 /*
 106  * os_refgrp_decl(qual, var, name, parent): declare a refgroup object 'var' with
 107  *   given name string and parent group.
 108  */
 109
 110 /*
 111  *
 112  * os_ref_retain: acquire a reference (increment reference count by 1) atomically.
 113  *
 114  * os_ref_release: release a reference (decrement reference count) atomically and
 115  *              return the new count. Memory is synchronized such that the dealloc block
 116  *              (i.e. code handling the final release() == 0 call) sees up-to-date memory
 117  *              with respect to all prior release()s on the same refcnt object. This
 118  *              memory ordering is sufficient for most use cases.
 119  *
 120  * os_ref_release_relaxed: same as release() but with weaker relaxed memory ordering.
 121  *              This can be used when the dealloc block is already synchronized with other
 122  *              accesses to the object (for example, with a lock).
 123  *
 124  * os_ref_release_live: release a reference that is guaranteed not to be the last one.
 125  */
 126 static void os_ref_retain(struct os_refcnt *);
 127 static os_ref_count_t os_ref_release(struct os_refcnt *) OS_WARN_RESULT;
 128 static os_ref_count_t os_ref_release_relaxed(struct os_refcnt *) OS_WARN_RESULT;
 129 static void os_ref_release_live(struct os_refcnt *);
 130
 131 /*
 132  * os_ref_retain_try: a variant of atomic retain that fails for objects with a
 133  *              zero reference count. The caller must therefore ensure that the object
 134  *              remains alive for any possible retain_try() caller, usually by using a
 135  *              lock protecting both the retain and dealloc paths. This variant is useful
 136  *              for objects stored in a collection, because no lock is required on the
 137  *              release() side until the object is deallocated.
 138  */
 139 static bool os_ref_retain_try(struct os_refcnt *) OS_WARN_RESULT;
 140
 141 /*
 142  * os_ref_retain_locked: acquire a reference on an object protected by a held
 143  *              lock. The caller must ensure mutual exclusivity of retain_locked() and
 144  *              release_locked() calls on the same object.
 145  *
 146  * os_ref_release_locked: release a reference on an object protected by a held
 147  *              lock.
 148  */
 149 static void os_ref_retain_locked(struct os_refcnt *);
 150 static os_ref_count_t os_ref_release_locked(struct os_refcnt *) OS_WARN_RESULT;
 151
 152 /*
 153  * os_ref_get_count: return the current reference count. This is unsafe for
 154  *              synchronization.
 155  */
 156 static os_ref_count_t os_ref_get_count(struct os_refcnt *rc);
 157
 158
 159 #if XNU_KERNEL_PRIVATE
 160 /*
 161  * Raw API that uses a plain atomic counter (os_ref_atomic_t) and a separate
 162  * refgroup. This can be used in situations where the refcount object must be
 163  * fixed size, for example for embedding in structures with ABI stability
 164  * requirements.
 165  */
 166
 167 #define os_ref_init_raw(rc, grp) os_ref_init_count_raw((rc), (grp), 1)
 168 static void os_ref_init_count_raw(os_ref_atomic_t *, struct os_refgrp *, os_ref_count_t count)
 169 os_error_if(count == 0, "Reference count must be non-zero initialized");
 170 static void os_ref_retain_raw(os_ref_atomic_t *, struct os_refgrp *);
 171 static os_ref_count_t os_ref_release_raw(os_ref_atomic_t *, struct os_refgrp *) OS_WARN_RESULT;
 172 static os_ref_count_t os_ref_release_relaxed_raw(os_ref_atomic_t *, struct os_refgrp *) OS_WARN_RESULT;
 173 static void os_ref_release_live_raw(os_ref_atomic_t *, struct os_refgrp *);
 174 static bool os_ref_retain_try_raw(os_ref_atomic_t *, struct os_refgrp *) OS_WARN_RESULT;
 175 static void os_ref_retain_locked_raw(os_ref_atomic_t *, struct os_refgrp *);
 176 static os_ref_count_t os_ref_release_locked_raw(os_ref_atomic_t *, struct os_refgrp *) OS_WARN_RESULT;
 177 static os_ref_count_t os_ref_get_count_raw(os_ref_atomic_t *rc);
 178
 179
 180 /*
 181  * Bitwise API: like the raw API, but allows some bits in the refcount value to be
 182  * reserved for other purposes. 'b' defines the number of trailing (LSB) reserved
 183  * bits, which the refcnt_raw API will never modify (except at init()).
 184  *
 185  * It is assumed that users of this API always use atomic ops on the
 186  * os_ref_atomic_t (or hold a lock for the locked variants), and never modify the
 187  * top (32 - 'b') bits.
 188  *
 189  * Due to guard bits, the maximum reference count is 2^(28 - 'b') - 1, and the
 190  * maximum 'b' is 26 bits. This API can also be used just to limit the max
 191  * refcount.
 192  */
 193
 194 /* Initialize the reference count and reserved bits */
 195 #define os_ref_init_mask(rc, grp, b) os_ref_init_count_mask((rc), (grp), 1, 0, (b))
 196 void os_ref_init_count_mask(os_ref_atomic_t *rc, struct os_refgrp *grp, os_ref_count_t init_count,
 197     os_ref_count_t init_bits, os_ref_count_t b)
 198 os_error_if(init_count == 0, "Reference count must be non-zero initialized")
 199 os_error_if(b > 26, "Bitwise reference count limited to 26 bits")
 200 os_error_if(init_bits >= (1U << b), "Bits out of range");
 201
 202 void os_ref_retain_mask(os_ref_atomic_t *rc, struct os_refgrp *grp, os_ref_count_t b);
 203 static os_ref_count_t os_ref_release_mask(os_ref_atomic_t *rc, struct os_refgrp *grp, os_ref_count_t b) OS_WARN_RESULT;
 204 static os_ref_count_t os_ref_release_relaxed_mask(os_ref_atomic_t *rc, struct os_refgrp *grp, os_ref_count_t b) OS_WARN_RESULT;
 205 static void os_ref_release_live_mask(os_ref_atomic_t *rc, struct os_refgrp *grp, os_ref_count_t b);
 206 bool os_ref_retain_try_mask(os_ref_atomic_t *, struct os_refgrp *grp, os_ref_count_t b) OS_WARN_RESULT;
 207 void os_ref_retain_locked_mask(os_ref_atomic_t *rc, struct os_refgrp *grp, os_ref_count_t b);
 208 os_ref_count_t os_ref_release_locked_mask(os_ref_atomic_t *rc, struct os_refgrp *grp, os_ref_count_t b) OS_WARN_RESULT;
 209 os_ref_count_t os_ref_get_count_mask(os_ref_atomic_t *rc, os_ref_count_t b);
 210
 211 #endif /* XNU_KERNEL_PRIVATE */
 212
 213 __END_DECLS
 214
 215 #include <os/refcnt_internal.h>
 216 #endif