diff options
Diffstat (limited to 'src/backend/port/posix_sema.c')
| -rw-r--r-- | src/backend/port/posix_sema.c | 37 |
1 files changed, 5 insertions, 32 deletions
diff --git a/src/backend/port/posix_sema.c b/src/backend/port/posix_sema.c index 7eaa89cdddb..335dae270d0 100644 --- a/src/backend/port/posix_sema.c +++ b/src/backend/port/posix_sema.c @@ -11,7 +11,7 @@ * Portions Copyright (c) 1994, Regents of the University of California * * IDENTIFICATION - * $PostgreSQL: pgsql/src/backend/port/posix_sema.c,v 1.19 2008/01/01 19:45:51 momjian Exp $ + * $PostgreSQL: pgsql/src/backend/port/posix_sema.c,v 1.20 2008/01/26 19:55:08 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -241,37 +241,10 @@ PGSemaphoreLock(PGSemaphore sema, bool interruptOK) int errStatus; /* - * Note: if errStatus is -1 and errno == EINTR then it means we returned - * from the operation prematurely because we were sent a signal. So we - * try and lock the semaphore again. - * - * Each time around the loop, we check for a cancel/die interrupt. We - * assume that if such an interrupt comes in while we are waiting, it will - * cause the sem_wait() call to exit with errno == EINTR, so that we will - * be able to service the interrupt (if not in a critical section - * already). - * - * Once we acquire the lock, we do NOT check for an interrupt before - * returning. The caller needs to be able to record ownership of the lock - * before any interrupt can be accepted. - * - * There is a window of a few instructions between CHECK_FOR_INTERRUPTS - * and entering the sem_wait() call. If a cancel/die interrupt occurs in - * that window, we would fail to notice it until after we acquire the lock - * (or get another interrupt to escape the sem_wait()). We can avoid this - * problem by temporarily setting ImmediateInterruptOK to true before we - * do CHECK_FOR_INTERRUPTS; then, a die() interrupt in this interval will - * execute directly. However, there is a huge pitfall: there is another - * window of a few instructions after the sem_wait() before we are able to - * reset ImmediateInterruptOK. If an interrupt occurs then, we'll lose - * control, which means that the lock has been acquired but our caller did - * not get a chance to record the fact. Therefore, we only set - * ImmediateInterruptOK if the caller tells us it's OK to do so, ie, the - * caller does not need to record acquiring the lock. (This is currently - * true for lockmanager locks, since the process that granted us the lock - * did all the necessary state updates. It's not true for Posix semaphores - * used to implement LW locks or emulate spinlocks --- but the wait time - * for such locks should not be very long, anyway.) + * See notes in sysv_sema.c's implementation of PGSemaphoreLock. + * Just as that code does for semop(), we handle both the case where + * sem_wait() returns errno == EINTR after a signal, and the case + * where it just keeps waiting. */ do { |