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Avoid using rand() that leads to a lock contention #804

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Jan 8, 2024
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Avoid using rand() that leads to a lock contention #804

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9 changes: 2 additions & 7 deletions src/shims/yield.h
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Original file line number Diff line number Diff line change
Expand Up @@ -98,22 +98,17 @@ void *_dispatch_wait_for_enqueuer(void **ptr);
#define _dispatch_contention_spins() \
((DISPATCH_CONTENTION_SPINS_MIN) + ((DISPATCH_CONTENTION_SPINS_MAX) - \
(DISPATCH_CONTENTION_SPINS_MIN)) / 2)
#elif defined(_WIN32)
#else
// Use randomness to prevent threads from resonating at the same frequency and
// permanently contending. Windows doesn't provide rand_r(), so use a simple
// LCG. (msvcrt has rand_s(), but its security guarantees aren't optimal here.)
// The implementation of rand() can contain a lock (known with glibc at least).
#define _dispatch_contention_spins() ({ \
static os_atomic(unsigned int) _seed = 1; \
unsigned int _next = os_atomic_load(&_seed, relaxed); \
os_atomic_store(&_seed, _next * 1103515245 + 12345, relaxed); \
((_next >> 24) & (DISPATCH_CONTENTION_SPINS_MAX)) | \
(DISPATCH_CONTENTION_SPINS_MIN); })
#else
// Use randomness to prevent threads from resonating at the same
// frequency and permanently contending.
#define _dispatch_contention_spins() ({ \
((unsigned int)rand() & (DISPATCH_CONTENTION_SPINS_MAX)) | \
(DISPATCH_CONTENTION_SPINS_MIN); })
#endif
#define _dispatch_contention_wait_until(c) ({ \
bool _out = false; \
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