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
  29 #include "c11/threads.h"
  30
  31 #if defined(__GNUC__) && defined(HAVE_LINUX_FUTEX_H)
  32
  33 /* mtx_t - Fast, simple mutex
  34  *
  35  * While modern pthread mutexes are very fast (implemented using futex), they
  36  * still incur a call to an external DSO and overhead of the generality and
  37  * features of pthread mutexes.  Most mutexes in mesa only needs lock/unlock,
  38  * and the idea here is that we can inline the atomic operation and make the
  39  * fast case just two intructions.  Mutexes are subtle and finicky to
  40  * implement, so we carefully copy the implementation from Ulrich Dreppers
  41  * well-written and well-reviewed paper:
  42  *
  43  *   "Futexes Are Tricky"
  44  *   http://www.akkadia.org/drepper/futex.pdf
  45  *
  46  * We implement "mutex3", which gives us a mutex that has no syscalls on
  47  * uncontended lock or unlock.  Further, the uncontended case boils down to a
  48  * locked cmpxchg and an untaken branch, the uncontended unlock is just a
  49  * locked decr and an untaken branch.  We use __builtin_expect() to indicate
  50  * that contention is unlikely so that gcc will put the contention code out of
  51  * the main code flow.
  52  *
  53  * A fast mutex only supports lock/unlock, can't be recursive or used with
  54  * condition variables.
  55  */
  56
  57 typedef struct {
  58    uint32_t val;
  59 } simple_mtx_t;
  60
  61 #define _SIMPLE_MTX_INITIALIZER_NP { 0 }
  62
  63 static inline void
  64 simple_mtx_init(simple_mtx_t *mtx, ASSERTED int type)
  65 {
  66    assert(type == mtx_plain);
  67
  68    mtx->val = 0;
  69 }
  70
  71 static inline void
  72 simple_mtx_destroy(UNUSED simple_mtx_t *mtx)
  73 {
  74 }
  75
  76 static inline void
  77 simple_mtx_lock(simple_mtx_t *mtx)
  78 {
  79    uint32_t c;
  80
  81    c = __sync_val_compare_and_swap(&mtx->val, 0, 1);
  82    if (__builtin_expect(c != 0, 0)) {
  83       if (c != 2)
  84          c = __sync_lock_test_and_set(&mtx->val, 2);
  85       while (c != 0) {
  86          futex_wait(&mtx->val, 2, NULL);
  87          c = __sync_lock_test_and_set(&mtx->val, 2);
  88       }
  89    }
  90 }
  91
  92 static inline void
  93 simple_mtx_unlock(simple_mtx_t *mtx)
  94 {
  95    uint32_t c;
  96
  97    c = __sync_fetch_and_sub(&mtx->val, 1);
  98    if (__builtin_expect(c != 1, 0)) {
  99       mtx->val = 0;
 100       futex_wake(&mtx->val, 1);
 101    }
 102 }
 103
 104 #else
 105
 106 typedef mtx_t simple_mtx_t;
 107
 108 #define _SIMPLE_MTX_INITIALIZER_NP _MTX_INITIALIZER_NP
 109
 110 static inline void
 111 simple_mtx_init(simple_mtx_t *mtx, int type)
 112 {
 113    mtx_init(mtx, type);
 114 }
 115
 116 static inline void
 117 simple_mtx_destroy(simple_mtx_t *mtx)
 118 {
 119    mtx_destroy(mtx);
 120 }
 121
 122 static inline void
 123 simple_mtx_lock(simple_mtx_t *mtx)
 124 {
 125    mtx_lock(mtx);
 126 }
 127
 128 static inline void
 129 simple_mtx_unlock(simple_mtx_t *mtx)
 130 {
 131    mtx_unlock(mtx);
 132 }
 133
 134 #endif
 135
 136 #endif