src/util/simple_mtx.h

   1 /*
   2  * Copyright © 2015 Intel
   3  *
   4  * Permission is hereby granted, free of charge, to any person obtaining a
   5  * copy of this software and associated documentation files (the "Software"),
   6  * to deal in the Software without restriction, including without limitation
   7  * the rights to use, copy, modify, merge, publish, distribute, sublicense,
   8  * and/or sell copies of the Software, and to permit persons to whom the
   9  * Software is furnished to do so, subject to the following conditions:
  10  *
  11  * The above copyright notice and this permission notice (including the next
  12  * paragraph) shall be included in all copies or substantial portions of the
  13  * Software.
  14  *
  15  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  16  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  17  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
  18  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  19  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
  20  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
  21  * IN THE SOFTWARE.
  22  */
  23
  24 #ifndef _SIMPLE_MTX_H
  25 #define _SIMPLE_MTX_H
  26
  27 #include "util/futex.h"
  28 #include "util/macros.h"
  29
  30 #include "c11/threads.h"
  31
  32 #if defined(__GNUC__) && defined(HAVE_LINUX_FUTEX_H)
  33
  34 /* mtx_t - Fast, simple mutex
  35  *
  36  * While modern pthread mutexes are very fast (implemented using futex), they
  37  * still incur a call to an external DSO and overhead of the generality and
  38  * features of pthread mutexes.  Most mutexes in mesa only needs lock/unlock,
  39  * and the idea here is that we can inline the atomic operation and make the
  40  * fast case just two intructions.  Mutexes are subtle and finicky to
  41  * implement, so we carefully copy the implementation from Ulrich Dreppers
  42  * well-written and well-reviewed paper:
  43  *
  44  *   "Futexes Are Tricky"
  45  *   http://www.akkadia.org/drepper/futex.pdf
  46  *
  47  * We implement "mutex3", which gives us a mutex that has no syscalls on
  48  * uncontended lock or unlock.  Further, the uncontended case boils down to a
  49  * locked cmpxchg and an untaken branch, the uncontended unlock is just a
  50  * locked decr and an untaken branch.  We use __builtin_expect() to indicate
  51  * that contention is unlikely so that gcc will put the contention code out of
  52  * the main code flow.
  53  *
  54  * A fast mutex only supports lock/unlock, can't be recursive or used with
  55  * condition variables.
  56  */
  57
  58 typedef struct {
  59    uint32_t val;
  60 } simple_mtx_t;
  61
  62 #define _SIMPLE_MTX_INITIALIZER_NP { 0 }
  63
  64 #define _SIMPLE_MTX_INVALID_VALUE 0xd0d0d0d0
  65
  66 static inline void
  67 simple_mtx_init(simple_mtx_t *mtx, ASSERTED int type)
  68 {
  69    assert(type == mtx_plain);
  70
  71    mtx->val = 0;
  72 }
  73
  74 static inline void
  75 simple_mtx_destroy(ASSERTED simple_mtx_t *mtx)
  76 {
  77 #ifndef NDEBUG
  78    mtx->val = _SIMPLE_MTX_INVALID_VALUE;
  79 #endif
  80 }
  81
  82 static inline void
  83 simple_mtx_lock(simple_mtx_t *mtx)
  84 {
  85    uint32_t c;
  86
  87    c = __sync_val_compare_and_swap(&mtx->val, 0, 1);
  88
  89    assert(c != _SIMPLE_MTX_INVALID_VALUE);
  90
  91    if (__builtin_expect(c != 0, 0)) {
  92       if (c != 2)
  93          c = __sync_lock_test_and_set(&mtx->val, 2);
  94       while (c != 0) {
  95          futex_wait(&mtx->val, 2, NULL);
  96          c = __sync_lock_test_and_set(&mtx->val, 2);
  97       }
  98    }
  99 }
 100
 101 static inline void
 102 simple_mtx_unlock(simple_mtx_t *mtx)
 103 {
 104    uint32_t c;
 105
 106    c = __sync_fetch_and_sub(&mtx->val, 1);
 107
 108    assert(c != _SIMPLE_MTX_INVALID_VALUE);
 109
 110    if (__builtin_expect(c != 1, 0)) {
 111       mtx->val = 0;
 112       futex_wake(&mtx->val, 1);
 113    }
 114 }
 115
 116 static inline void
 117 simple_mtx_assert_locked(simple_mtx_t *mtx)
 118 {
 119    assert(mtx->val);
 120 }
 121
 122 #else
 123
 124 typedef mtx_t simple_mtx_t;
 125
 126 #define _SIMPLE_MTX_INITIALIZER_NP _MTX_INITIALIZER_NP
 127
 128 static inline void
 129 simple_mtx_init(simple_mtx_t *mtx, int type)
 130 {
 131    mtx_init(mtx, type);
 132 }
 133
 134 static inline void
 135 simple_mtx_destroy(simple_mtx_t *mtx)
 136 {
 137    mtx_destroy(mtx);
 138 }
 139
 140 static inline void
 141 simple_mtx_lock(simple_mtx_t *mtx)
 142 {
 143    mtx_lock(mtx);
 144 }
 145
 146 static inline void
 147 simple_mtx_unlock(simple_mtx_t *mtx)
 148 {
 149    mtx_unlock(mtx);
 150 }
 151
 152 static inline void
 153 simple_mtx_assert_locked(simple_mtx_t *mtx)
 154 {
 155 #ifdef DEBUG
 156    /* NOTE: this would not work for recursive mutexes, but
 157     * mtx_t doesn't support those
 158     */
 159    int ret = mtx_trylock(mtx);
 160    assert(ret == thrd_busy);
 161    if (ret == thrd_success)
 162       mtx_unlock(mtx);
 163 #else
 164    (void)mtx;
 165 #endif
 166 }
 167
 168 #endif
 169
 170 #endif