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/*
* Copyright © 2015 Intel
*
* Permission is hereby granted, free of charge, to any person obtaining a
* copy of this software and associated documentation files (the "Software"),
* to deal in the Software without restriction, including without limitation
* the rights to use, copy, modify, merge, publish, distribute, sublicense,
* and/or sell copies of the Software, and to permit persons to whom the
* Software is furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice (including the next
* paragraph) shall be included in all copies or substantial portions of the
* Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
* IN THE SOFTWARE.
*/
#ifndef _SIMPLE_MTX_H
#define _SIMPLE_MTX_H
#include "util/futex.h"
#include "c11/threads.h"
#if defined(__GNUC__) && defined(HAVE_LINUX_FUTEX_H)
/* mtx_t - Fast, simple mutex
*
* While modern pthread mutexes are very fast (implemented using futex), they
* still incur a call to an external DSO and overhead of the generality and
* features of pthread mutexes. Most mutexes in mesa only needs lock/unlock,
* and the idea here is that we can inline the atomic operation and make the
* fast case just two intructions. Mutexes are subtle and finicky to
* implement, so we carefully copy the implementation from Ulrich Dreppers
* well-written and well-reviewed paper:
*
* "Futexes Are Tricky"
* http://www.akkadia.org/drepper/futex.pdf
*
* We implement "mutex3", which gives us a mutex that has no syscalls on
* uncontended lock or unlock. Further, the uncontended case boils down to a
* locked cmpxchg and an untaken branch, the uncontended unlock is just a
* locked decr and an untaken branch. We use __builtin_expect() to indicate
* that contention is unlikely so that gcc will put the contention code out of
* the main code flow.
*
* A fast mutex only supports lock/unlock, can't be recursive or used with
* condition variables.
*/
typedef struct {
uint32_t val;
} simple_mtx_t;
#define _SIMPLE_MTX_INITIALIZER_NP { 0 }
static inline void
simple_mtx_init(simple_mtx_t *mtx, int type)
{
assert(type == mtx_plain);
mtx->val = 0;
}
static inline void
simple_mtx_destroy(simple_mtx_t *mtx)
{
}
static inline void
simple_mtx_lock(simple_mtx_t *mtx)
{
uint32_t c;
c = __sync_val_compare_and_swap(&mtx->val, 0, 1);
if (__builtin_expect(c != 0, 0)) {
if (c != 2)
c = __sync_lock_test_and_set(&mtx->val, 2);
while (c != 0) {
futex_wait(&mtx->val, 2, NULL);
c = __sync_lock_test_and_set(&mtx->val, 2);
}
}
}
static inline void
simple_mtx_unlock(simple_mtx_t *mtx)
{
uint32_t c;
c = __sync_fetch_and_sub(&mtx->val, 1);
if (__builtin_expect(c != 1, 0)) {
mtx->val = 0;
futex_wake(&mtx->val, 1);
}
}
#else
typedef mtx_t simple_mtx_t;
#define _SIMPLE_MTX_INITIALIZER_NP _MTX_INITIALIZER_NP
static inline void
simple_mtx_init(simple_mtx_t *mtx, int type)
{
mtx_init(mtx, type);
}
static inline void
simple_mtx_destroy(simple_mtx_t *mtx)
{
mtx_destroy(mtx);
}
static inline void
simple_mtx_lock(simple_mtx_t *mtx)
{
mtx_lock(mtx);
}
static inline void
simple_mtx_unlock(simple_mtx_t *mtx)
{
mtx_unlock(mtx);
}
#endif
#endif
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