From cc583082f4fee50dfe86e0f78298ed34b18feb18 Mon Sep 17 00:00:00 2001 From: David Xu Date: Wed, 21 Mar 2012 07:12:52 +0000 Subject: [PATCH] Use version 2 of semaphore provided by kernel umtx code, now if there is no waiters, we still increase and decrease count in user mode without entering kernel, once there is a waiter, sem_post will enter kernel to increase count and wake thread up, this is atomicy and allow us to gracefully destroy semaphore after sem_wait returned. --- lib/libc/gen/sem_new.c | 99 +++++++++++++++++++++++++----------------- 1 file changed, 60 insertions(+), 39 deletions(-) diff --git a/lib/libc/gen/sem_new.c b/lib/libc/gen/sem_new.c index af64310c6d8f..f0d97e6f650f 100644 --- a/lib/libc/gen/sem_new.c +++ b/lib/libc/gen/sem_new.c @@ -61,7 +61,8 @@ __weak_reference(_sem_unlink, sem_unlink); __weak_reference(_sem_wait, sem_wait); #define SEM_PREFIX "/tmp/SEMD" -#define SEM_MAGIC ((u_int32_t)0x73656d31) +#define SEM_MAGIC1 ((u_int32_t)0x73656d31) +#define SEM_MAGIC ((u_int32_t)0x73656d32) struct sem_nameinfo { int open_count; @@ -109,7 +110,7 @@ static inline int sem_check_validity(sem_t *sem) { - if (sem->_magic == SEM_MAGIC) + if (sem->_magic == SEM_MAGIC || sem->_magic == SEM_MAGIC1) return (0); else { errno = EINVAL; @@ -130,7 +131,7 @@ _sem_init(sem_t *sem, int pshared, unsigned int value) sem->_magic = SEM_MAGIC; sem->_kern._count = (u_int32_t)value; sem->_kern._has_waiters = 0; - sem->_kern._flags = pshared ? USYNC_PROCESS_SHARED : 0; + sem->_kern._flags = (pshared ? USYNC_PROCESS_SHARED : 0) | SEM_VER2; return (0); } @@ -207,7 +208,7 @@ _sem_open(const char *name, int flags, ...) tmp._magic = SEM_MAGIC; tmp._kern._has_waiters = 0; tmp._kern._count = value; - tmp._kern._flags = USYNC_PROCESS_SHARED | SEM_NAMED; + tmp._kern._flags = USYNC_PROCESS_SHARED | SEM_NAMED | SEM_VER2; if (_write(fd, &tmp, sizeof(tmp)) != sizeof(tmp)) { flock(fd, LOCK_UN); goto error; @@ -325,19 +326,10 @@ _sem_getvalue(sem_t * __restrict sem, int * __restrict sval) if (sem_check_validity(sem) != 0) return (-1); - *sval = (int)sem->_kern._count; + *sval = (int)sem->_kern._count & ~SEM_WAITERS; return (0); } -static __inline int -usem_wake(struct _usem *sem) -{ - rmb(); - if (!sem->_has_waiters) - return (0); - return _umtx_op(sem, UMTX_OP_SEM_WAKE, 0, NULL, NULL); -} - static __inline int usem_wait(struct _usem *sem, const struct timespec *abstime) { @@ -358,48 +350,51 @@ usem_wait(struct _usem *sem, const struct timespec *abstime) (void *)tm_size, __DECONST(void*, tm_p)); } +static inline int +_trywait(sem_t *sem) +{ + int count; + + if ((sem->_kern._flags & SEM_VER2) != 0) { + while (((count = sem->_kern._count) & ~SEM_WAITERS) > 0) { + if (atomic_cmpset_acq_int(&sem->_kern._count, count, count - 1)) + return (0); + } + } else { + while ((count = sem->_kern._count) > 0) { + if (atomic_cmpset_acq_int(&sem->_kern._count, count, count - 1)) + return (0); + } + } + return (EAGAIN); +} + int _sem_trywait(sem_t *sem) { - int val; + int status; if (sem_check_validity(sem) != 0) return (-1); - - while ((val = sem->_kern._count) > 0) { - if (atomic_cmpset_acq_int(&sem->_kern._count, val, val - 1)) - return (0); - } - errno = EAGAIN; + if ((status = _trywait(sem)) == 0) + return (0); + errno = status; return (-1); } -#define TIMESPEC_SUB(dst, src, val) \ - do { \ - (dst)->tv_sec = (src)->tv_sec - (val)->tv_sec; \ - (dst)->tv_nsec = (src)->tv_nsec - (val)->tv_nsec; \ - if ((dst)->tv_nsec < 0) { \ - (dst)->tv_sec--; \ - (dst)->tv_nsec += 1000000000; \ - } \ - } while (0) - - int _sem_timedwait(sem_t * __restrict sem, const struct timespec * __restrict abstime) { - int val, retval; + int retval; if (sem_check_validity(sem) != 0) return (-1); retval = 0; for (;;) { - while ((val = sem->_kern._count) > 0) { - if (atomic_cmpset_acq_int(&sem->_kern._count, val, val - 1)) - return (0); - } + if (_trywait(sem) == 0) + return (0); if (retval) { _pthread_testcancel(); @@ -438,10 +433,36 @@ _sem_wait(sem_t *sem) int _sem_post(sem_t *sem) { + int count; if (sem_check_validity(sem) != 0) return (-1); - atomic_add_rel_int(&sem->_kern._count, 1); - return usem_wake(&sem->_kern); + if ((sem->_kern._flags & SEM_VER2) != 0) { + for (;;) { + count = sem->_kern._count; + if ((count & SEM_WAITERS) == 0) { + if (__predict_false(count == SEM_VALUE_MAX)) { + errno = ERANGE; + return (-1); + } + if (atomic_cmpset_rel_int(&sem->_kern._count, count, count+1)) + return (0); + } else { + return _umtx_op(&sem->_kern, UMTX_OP_SEM_WAKE, 0, NULL, NULL); + } + } + } else { + do { + count = sem->_kern._count; + if (__predict_false(count == SEM_VALUE_MAX)) { + errno = ERANGE; + return (-1); + } + } while (!atomic_cmpset_rel_int(&sem->_kern._count, count, count+1)); + rmb(); + if (!sem->_kern._has_waiters) + return (0); + return _umtx_op(&sem->_kern, UMTX_OP_SEM_WAKE, 0, NULL, NULL); + } }