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148 lines
3.8 KiB
C
148 lines
3.8 KiB
C
/*
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* Copyright © 2015 Intel
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*
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* Permission is hereby granted, free of charge, to any person obtaining a
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* copy of this software and associated documentation files (the "Software"),
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* to deal in the Software without restriction, including without limitation
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* the rights to use, copy, modify, merge, publish, distribute, sublicense,
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* and/or sell copies of the Software, and to permit persons to whom the
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* Software is furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice (including the next
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* paragraph) shall be included in all copies or substantial portions of the
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* Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
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* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
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* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
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* IN THE SOFTWARE.
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*/
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#ifndef _SIMPLE_MTX_H
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#define _SIMPLE_MTX_H
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#include "../util/futex.h"
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#include "../c11/threads.h"
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#if defined(__GNUC__) && defined(HAVE_LINUX_FUTEX_H)
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/* mtx_t - Fast, simple mutex
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*
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* While modern pthread mutexes are very fast (implemented using futex), they
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* still incur a call to an external DSO and overhead of the generality and
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* features of pthread mutexes. Most mutexes in mesa only needs lock/unlock,
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* and the idea here is that we can inline the atomic operation and make the
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* fast case just two intructions. Mutexes are subtle and finicky to
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* implement, so we carefully copy the implementation from Ulrich Dreppers
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* well-written and well-reviewed paper:
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*
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* "Futexes Are Tricky"
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* http://www.akkadia.org/drepper/futex.pdf
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*
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* We implement "mutex3", which gives us a mutex that has no syscalls on
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* uncontended lock or unlock. Further, the uncontended case boils down to a
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* locked cmpxchg and an untaken branch, the uncontended unlock is just a
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* locked decr and an untaken branch. We use __builtin_expect() to indicate
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* that contention is unlikely so that gcc will put the contention code out of
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* the main code flow.
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*
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* A fast mutex only supports lock/unlock, can't be recursive or used with
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* condition variables.
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*/
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typedef struct {
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uint32_t val;
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} simple_mtx_t;
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#define _SIMPLE_MTX_INITIALIZER_NP { 0 }
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#define _SIMPLE_MTX_INVALID_VALUE 0xd0d0d0d0
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static inline void
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simple_mtx_init(simple_mtx_t *mtx, ASSERTED int type)
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{
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assert(type == mtx_plain);
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mtx->val = 0;
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}
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static inline void
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simple_mtx_destroy(ASSERTED simple_mtx_t *mtx)
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{
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#ifndef NDEBUG
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mtx->val = _SIMPLE_MTX_INVALID_VALUE;
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#endif
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}
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static inline void
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simple_mtx_lock(simple_mtx_t *mtx)
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{
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uint32_t c;
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c = __sync_val_compare_and_swap(&mtx->val, 0, 1);
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assert(c != _SIMPLE_MTX_INVALID_VALUE);
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if (__builtin_expect(c != 0, 0)) {
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if (c != 2)
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c = __sync_lock_test_and_set(&mtx->val, 2);
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while (c != 0) {
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futex_wait(&mtx->val, 2, NULL);
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c = __sync_lock_test_and_set(&mtx->val, 2);
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}
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}
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}
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static inline void
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simple_mtx_unlock(simple_mtx_t *mtx)
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{
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uint32_t c;
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c = __sync_fetch_and_sub(&mtx->val, 1);
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assert(c != _SIMPLE_MTX_INVALID_VALUE);
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if (__builtin_expect(c != 1, 0)) {
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mtx->val = 0;
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futex_wake(&mtx->val, 1);
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}
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}
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#else
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typedef mtx_t simple_mtx_t;
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#define _SIMPLE_MTX_INITIALIZER_NP _MTX_INITIALIZER_NP
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static inline void
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simple_mtx_init(simple_mtx_t *mtx, int type)
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{
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mtx_init(mtx, type);
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}
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static inline void
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simple_mtx_destroy(simple_mtx_t *mtx)
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{
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mtx_destroy(mtx);
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}
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static inline void
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simple_mtx_lock(simple_mtx_t *mtx)
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{
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mtx_lock(mtx);
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}
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static inline void
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simple_mtx_unlock(simple_mtx_t *mtx)
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{
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mtx_unlock(mtx);
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}
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#endif
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#endif
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