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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 }
63 #define _SIMPLE_MTX_INVALID_VALUE 0xd0d0d0d0
66 simple_mtx_init(simple_mtx_t
*mtx
, ASSERTED
int type
)
68 assert(type
== mtx_plain
);
74 simple_mtx_destroy(simple_mtx_t
*mtx
)
76 mtx
->val
= _SIMPLE_MTX_INVALID_VALUE
;
80 simple_mtx_lock(simple_mtx_t
*mtx
)
84 c
= __sync_val_compare_and_swap(&mtx
->val
, 0, 1);
86 assert(c
!= _SIMPLE_MTX_INVALID_VALUE
);
88 if (__builtin_expect(c
!= 0, 0)) {
90 c
= __sync_lock_test_and_set(&mtx
->val
, 2);
92 futex_wait(&mtx
->val
, 2, NULL
);
93 c
= __sync_lock_test_and_set(&mtx
->val
, 2);
99 simple_mtx_unlock(simple_mtx_t
*mtx
)
103 c
= __sync_fetch_and_sub(&mtx
->val
, 1);
105 assert(c
!= _SIMPLE_MTX_INVALID_VALUE
);
107 if (__builtin_expect(c
!= 1, 0)) {
109 futex_wake(&mtx
->val
, 1);
115 typedef mtx_t simple_mtx_t
;
117 #define _SIMPLE_MTX_INITIALIZER_NP _MTX_INITIALIZER_NP
120 simple_mtx_init(simple_mtx_t
*mtx
, int type
)
126 simple_mtx_destroy(simple_mtx_t
*mtx
)
132 simple_mtx_lock(simple_mtx_t
*mtx
)
138 simple_mtx_unlock(simple_mtx_t
*mtx
)