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27 #include "util/futex.h"
29 #include "c11/threads.h"
31 #if defined(__GNUC__) && defined(HAVE_LINUX_FUTEX_H)
33 /* mtx_t - Fast, simple mutex
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:
43 * "Futexes Are Tricky"
44 * http://www.akkadia.org/drepper/futex.pdf
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
53 * A fast mutex only supports lock/unlock, can't be recursive or used with
54 * condition variables.
61 #define _SIMPLE_MTX_INITIALIZER_NP { 0 }
64 simple_mtx_init(simple_mtx_t
*mtx
, MAYBE_UNUSED
int type
)
66 assert(type
== mtx_plain
);
72 simple_mtx_destroy(UNUSED simple_mtx_t
*mtx
)
77 simple_mtx_lock(simple_mtx_t
*mtx
)
81 c
= __sync_val_compare_and_swap(&mtx
->val
, 0, 1);
82 if (__builtin_expect(c
!= 0, 0)) {
84 c
= __sync_lock_test_and_set(&mtx
->val
, 2);
86 futex_wait(&mtx
->val
, 2, NULL
);
87 c
= __sync_lock_test_and_set(&mtx
->val
, 2);
93 simple_mtx_unlock(simple_mtx_t
*mtx
)
97 c
= __sync_fetch_and_sub(&mtx
->val
, 1);
98 if (__builtin_expect(c
!= 1, 0)) {
100 futex_wake(&mtx
->val
, 1);
106 typedef mtx_t simple_mtx_t
;
108 #define _SIMPLE_MTX_INITIALIZER_NP _MTX_INITIALIZER_NP
111 simple_mtx_init(simple_mtx_t
*mtx
, int type
)
117 simple_mtx_destroy(simple_mtx_t
*mtx
)
123 simple_mtx_lock(simple_mtx_t
*mtx
)
129 simple_mtx_unlock(simple_mtx_t
*mtx
)