path: root/src/thread/sem_trywait.c
AgeCommit message (Collapse)AuthorLines
2012-07-24retry on cas failures in sem_trywaitRich Felker-2/+2
this seems counter-intuitive since sem_trywait is supposed to just try once, not wait for the semaphore. however, the retry loop is not a wait. instead, it's to handle the case where the value changes due to a simultaneous post or wait from another thread while the semaphore value remains positive. in such a case, it's absolutely wrong for sem_trywait to fail with EAGAIN because the semaphore is not busy.
2011-08-02overhaul posix semaphores to fix destructability raceRich Felker-1/+4
the race condition these changes address is described in glibc bug report number 12674: up until now, musl has shared the bug, and i had not been able to figure out how to eliminate it. in short, the problem is that it's not valid for sem_post to inspect the waiters count after incrementing the semaphore value, because another thread may have already successfully returned from sem_wait, (rightly) deemed itself the only remaining user of the semaphore, and chosen to destroy and free it (or unmap the shared memory it's stored in). POSIX is not explicit in blessing this usage, but it gives a very explicit analogous example with mutexes (which, in musl and glibc, also suffer from the same race condition bug) in the rationale for pthread_mutex_destroy. the new semaphore implementation augments the waiter count with a redundant waiter indication in the semaphore value itself, representing the presence of "last minute" waiters that may have arrived after sem_post read the waiter count. this allows sem_post to read the waiter count prior to incrementing the semaphore value, rather than after incrementing it, so as to avoid accessing the semaphore memory whatsoever after the increment takes place. a similar, but much simpler, fix should be possible for mutexes and other locking primitives whose usage rules are stricter than semaphores.
2011-04-14change sem_trywait algorithm so it never has to call __wakeRich Felker-3/+2
2011-04-06major semaphore improvements (performance and correctness)Rich Felker-3/+2
1. make sem_[timed]wait interruptible by signals, per POSIX 2. keep a waiter count in order to avoid unnecessary futex wake syscalls
2011-03-10fix some semaphore wait semantics (race condition deadlock and error checking)Rich Felker-1/+2
2011-03-04implement POSIX semaphoresRich Felker-0/+11