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LibPthread: Reimplement semaphores

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This implementation does not use locking or condition variables
internally; it's purely based on atomics and futexes.

Notably, concurrent sem_wait() and sem_post() calls can run *completely
in parallel* without slowing each other down, as long as there are empty
slots for them all to succeed without blocking.

Additionally, sem_wait() never executes an atomic operation with release
ordering, and sem_post() never executes an atomic operation with acquire
ordering (unless you count the syscall). This means the compiler and the
hardware are free to reorder code *into* the critical section.
This commit is contained in:
Sergey Bugaev 2021-07-05 15:10:34 +03:00 • committed by Andreas Kling
parent 00d8dbe739
commit 690141ff8b
2 changed files with 107 additions and 132 deletions
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5
Userland/Libraries/LibPthread/semaphore.h
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@ -14,9 +14,7 @@
__BEGIN_DECLS
typedef struct {
pthread_mutex_t mtx;
pthread_cond_t cv;
int value;
uint32_t value;
} sem_t;
int sem_close(sem_t*);
@ -28,6 +26,7 @@ int sem_post(sem_t*);
int sem_trywait(sem_t*);
int sem_unlink(const char*);
int sem_wait(sem_t*);
int sem_timedwait(sem_t*, const struct timespec* abstime);
#define SEM_VALUE_MAX INT_MAX