Author: jhb Date: Fri Oct 24 20:02:44 2014 New Revision: 273604 URL: https://svnweb.freebsd.org/changeset/base/273604
Log: The current POSIX semaphore implementation stores the _has_waiters flag in a separate word from the _count. This does not permit both items to be updated atomically in a portable manner. As a result, sem_post() must always perform a system call to safely clear _has_waiters. This change removes the _has_waiters field and instead uses the high bit of _count as the _has_waiters flag. A new umtx object type (_usem2) and two new umtx operations are added (SEM_WAIT2 and SEM_WAKE2) to implement these semantics. The older operations are still supported under the COMPAT_FREEBSD9/10 options. The POSIX semaphore API in libc has been updated to use the new implementation. Note that the new implementation is not compatible with the previous implementation. However, this only affects static binaries (which cannot be helped by symbol versioning). Binaries using a dynamic libc will continue to work fine. SEM_MAGIC has been bumped so that mismatched binaries will error rather than corrupting a shared semaphore. In addition, a padding field has been added to sem_t so that it remains the same size. Differential Revision: https://reviews.freebsd.org/D961 Reported by: adrian Reviewed by: kib, jilles (earlier version) Sponsored by: Norse Modified: head/include/semaphore.h head/lib/libc/gen/sem_new.c head/sys/kern/kern_umtx.c head/sys/sys/_umtx.h head/sys/sys/umtx.h head/usr.bin/truss/syscalls.c Modified: head/include/semaphore.h ============================================================================== --- head/include/semaphore.h Fri Oct 24 19:58:24 2014 (r273603) +++ head/include/semaphore.h Fri Oct 24 20:02:44 2014 (r273604) @@ -38,7 +38,8 @@ struct _sem { __uint32_t _magic; - struct _usem _kern; + struct _usem2 _kern; + __uint32_t _padding; /* Preserve structure size */ }; typedef struct _sem sem_t; Modified: head/lib/libc/gen/sem_new.c ============================================================================== --- head/lib/libc/gen/sem_new.c Fri Oct 24 19:58:24 2014 (r273603) +++ head/lib/libc/gen/sem_new.c Fri Oct 24 20:02:44 2014 (r273604) @@ -61,7 +61,9 @@ __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_MAGIC ((u_int32_t)0x73656d32) + +_Static_assert(SEM_VALUE_MAX <= USEM_MAX_COUNT, "SEM_VALUE_MAX too large"); struct sem_nameinfo { int open_count; @@ -131,7 +133,6 @@ _sem_init(sem_t *sem, int pshared, unsig bzero(sem, sizeof(sem_t)); sem->_magic = SEM_MAGIC; sem->_kern._count = (u_int32_t)value; - sem->_kern._has_waiters = 0; sem->_kern._flags = pshared ? USYNC_PROCESS_SHARED : 0; return (0); } @@ -212,7 +213,6 @@ _sem_open(const char *name, int flags, . sem_t tmp; tmp._magic = SEM_MAGIC; - tmp._kern._has_waiters = 0; tmp._kern._count = value; tmp._kern._flags = USYNC_PROCESS_SHARED | SEM_NAMED; if (_write(fd, &tmp, sizeof(tmp)) != sizeof(tmp)) @@ -332,18 +332,18 @@ _sem_getvalue(sem_t * __restrict sem, in if (sem_check_validity(sem) != 0) return (-1); - *sval = (int)sem->_kern._count; + *sval = (int)USEM_COUNT(sem->_kern._count); return (0); } static __inline int -usem_wake(struct _usem *sem) +usem_wake(struct _usem2 *sem) { - return _umtx_op(sem, UMTX_OP_SEM_WAKE, 0, NULL, NULL); + return _umtx_op(sem, UMTX_OP_SEM2_WAKE, 0, NULL, NULL); } static __inline int -usem_wait(struct _usem *sem, const struct timespec *abstime) +usem_wait(struct _usem2 *sem, const struct timespec *abstime) { struct _umtx_time *tm_p, timeout; size_t tm_size; @@ -358,7 +358,7 @@ usem_wait(struct _usem *sem, const struc tm_p = &timeout; tm_size = sizeof(timeout); } - return _umtx_op(sem, UMTX_OP_SEM_WAIT, 0, + return _umtx_op(sem, UMTX_OP_SEM2_WAIT, 0, (void *)tm_size, __DECONST(void*, tm_p)); } @@ -370,7 +370,7 @@ _sem_trywait(sem_t *sem) if (sem_check_validity(sem) != 0) return (-1); - while ((val = sem->_kern._count) > 0) { + while (USEM_COUNT(val = sem->_kern._count) > 0) { if (atomic_cmpset_acq_int(&sem->_kern._count, val, val - 1)) return (0); } @@ -390,7 +390,7 @@ _sem_timedwait(sem_t * __restrict sem, retval = 0; _pthread_testcancel(); for (;;) { - while ((val = sem->_kern._count) > 0) { + while (USEM_COUNT(val = sem->_kern._count) > 0) { if (atomic_cmpset_acq_int(&sem->_kern._count, val, val - 1)) return (0); } @@ -439,9 +439,10 @@ _sem_post(sem_t *sem) do { count = sem->_kern._count; - if (count + 1 > SEM_VALUE_MAX) + if (USEM_COUNT(count) + 1 > SEM_VALUE_MAX) return (EOVERFLOW); - } while(!atomic_cmpset_rel_int(&sem->_kern._count, count, count+1)); - (void)usem_wake(&sem->_kern); + } while (!atomic_cmpset_rel_int(&sem->_kern._count, count, count + 1)); + if (count & USEM_HAS_WAITERS) + usem_wake(&sem->_kern); return (0); } Modified: head/sys/kern/kern_umtx.c ============================================================================== --- head/sys/kern/kern_umtx.c Fri Oct 24 19:58:24 2014 (r273603) +++ head/sys/kern/kern_umtx.c Fri Oct 24 20:02:44 2014 (r273604) @@ -2710,6 +2710,7 @@ out: return (error); } +#if defined(COMPAT_FREEBSD9) || defined(COMPAT_FREEBSD10) static int do_sem_wait(struct thread *td, struct _usem *sem, struct _umtx_time *timeout) { @@ -2731,7 +2732,7 @@ do_sem_wait(struct thread *td, struct _u umtxq_busy(&uq->uq_key); umtxq_insert(uq); umtxq_unlock(&uq->uq_key); - casuword32(__DEVOLATILE(uint32_t *, &sem->_has_waiters), 0, 1); + casuword32(&sem->_has_waiters, 0, 1); count = fuword32(__DEVOLATILE(uint32_t *, &sem->_count)); if (count != 0) { umtxq_lock(&uq->uq_key); @@ -2761,7 +2762,7 @@ do_sem_wait(struct thread *td, struct _u } /* - * Signal a userland condition variable. + * Signal a userland semaphore. */ static int do_sem_wake(struct thread *td, struct _usem *sem) @@ -2795,6 +2796,119 @@ do_sem_wake(struct thread *td, struct _u umtx_key_release(&key); return (error); } +#endif + +static int +do_sem2_wait(struct thread *td, struct _usem2 *sem, struct _umtx_time *timeout) +{ + struct abs_timeout timo; + struct umtx_q *uq; + uint32_t count, flags; + int error; + + uq = td->td_umtxq; + flags = fuword32(&sem->_flags); + error = umtx_key_get(sem, TYPE_SEM, GET_SHARE(flags), &uq->uq_key); + if (error != 0) + return (error); + + if (timeout != NULL) + abs_timeout_init2(&timo, timeout); + + umtxq_lock(&uq->uq_key); + umtxq_busy(&uq->uq_key); + umtxq_insert(uq); + umtxq_unlock(&uq->uq_key); + count = fuword32(__DEVOLATILE(uint32_t *, &sem->_count)); + if (count == -1) { + umtxq_lock(&uq->uq_key); + umtxq_unbusy(&uq->uq_key); + umtxq_remove(uq); + umtxq_unlock(&uq->uq_key); + umtx_key_release(&uq->uq_key); + return (EFAULT); + } + for (;;) { + if (USEM_COUNT(count) != 0) { + umtxq_lock(&uq->uq_key); + umtxq_unbusy(&uq->uq_key); + umtxq_remove(uq); + umtxq_unlock(&uq->uq_key); + umtx_key_release(&uq->uq_key); + return (0); + } + if (count == USEM_HAS_WAITERS) + break; + count = casuword32(&sem->_count, 0, USEM_HAS_WAITERS); + if (count == -1) { + umtxq_lock(&uq->uq_key); + umtxq_unbusy(&uq->uq_key); + umtxq_remove(uq); + umtxq_unlock(&uq->uq_key); + umtx_key_release(&uq->uq_key); + return (EFAULT); + } + if (count == 0) + break; + } + umtxq_lock(&uq->uq_key); + umtxq_unbusy(&uq->uq_key); + + error = umtxq_sleep(uq, "usem", timeout == NULL ? NULL : &timo); + + if ((uq->uq_flags & UQF_UMTXQ) == 0) + error = 0; + else { + umtxq_remove(uq); + /* A relative timeout cannot be restarted. */ + if (error == ERESTART && timeout != NULL && + (timeout->_flags & UMTX_ABSTIME) == 0) + error = EINTR; + } + umtxq_unlock(&uq->uq_key); + umtx_key_release(&uq->uq_key); + return (error); +} + +/* + * Signal a userland semaphore. + */ +static int +do_sem2_wake(struct thread *td, struct _usem2 *sem) +{ + struct umtx_key key; + int error, cnt; + uint32_t count, flags; + + flags = fuword32(&sem->_flags); + if ((error = umtx_key_get(sem, TYPE_SEM, GET_SHARE(flags), &key)) != 0) + return (error); + umtxq_lock(&key); + umtxq_busy(&key); + cnt = umtxq_count(&key); + if (cnt > 0) { + umtxq_signal(&key, 1); + + /* + * If this was the last sleeping thread, clear the waiters + * flag in _count. + */ + if (cnt == 1) { + umtxq_unlock(&key); + count = fuword32(&sem->_count); + while (count != -1 && count & USEM_HAS_WAITERS) + count = casuword32(&sem->_count, count, + count & ~USEM_HAS_WAITERS); + if (count == -1) + error = EFAULT; + umtxq_lock(&key); + } + } + umtxq_unbusy(&key); + umtxq_unlock(&key); + umtx_key_release(&key); + return (error); +} inline int umtx_copyin_timeout(const void *addr, struct timespec *tsp) @@ -3066,6 +3180,7 @@ __umtx_op_rw_unlock(struct thread *td, s return do_rw_unlock(td, uap->obj); } +#if defined(COMPAT_FREEBSD9) || defined(COMPAT_FREEBSD10) static int __umtx_op_sem_wait(struct thread *td, struct _umtx_op_args *uap) { @@ -3090,6 +3205,7 @@ __umtx_op_sem_wake(struct thread *td, st { return do_sem_wake(td, uap->obj); } +#endif static int __umtx_op_wake2_umutex(struct thread *td, struct _umtx_op_args *uap) @@ -3097,6 +3213,31 @@ __umtx_op_wake2_umutex(struct thread *td return do_wake2_umutex(td, uap->obj, uap->val); } +static int +__umtx_op_sem2_wait(struct thread *td, struct _umtx_op_args *uap) +{ + struct _umtx_time *tm_p, timeout; + int error; + + /* Allow a null timespec (wait forever). */ + if (uap->uaddr2 == NULL) + tm_p = NULL; + else { + error = umtx_copyin_umtx_time( + uap->uaddr2, (size_t)uap->uaddr1, &timeout); + if (error != 0) + return (error); + tm_p = &timeout; + } + return (do_sem2_wait(td, uap->obj, tm_p)); +} + +static int +__umtx_op_sem2_wake(struct thread *td, struct _umtx_op_args *uap) +{ + return do_sem2_wake(td, uap->obj); +} + typedef int (*_umtx_op_func)(struct thread *td, struct _umtx_op_args *uap); static _umtx_op_func op_table[] = { @@ -3119,10 +3260,17 @@ static _umtx_op_func op_table[] = { __umtx_op_wake_private, /* UMTX_OP_WAKE_PRIVATE */ __umtx_op_wait_umutex, /* UMTX_OP_MUTEX_WAIT */ __umtx_op_wake_umutex, /* UMTX_OP_MUTEX_WAKE */ +#if defined(COMPAT_FREEBSD9) || defined(COMPAT_FREEBSD10) __umtx_op_sem_wait, /* UMTX_OP_SEM_WAIT */ __umtx_op_sem_wake, /* UMTX_OP_SEM_WAKE */ +#else + __umtx_op_unimpl, /* UMTX_OP_SEM_WAIT */ + __umtx_op_unimpl, /* UMTX_OP_SEM_WAKE */ +#endif __umtx_op_nwake_private, /* UMTX_OP_NWAKE_PRIVATE */ - __umtx_op_wake2_umutex /* UMTX_OP_MUTEX_WAKE2 */ + __umtx_op_wake2_umutex, /* UMTX_OP_MUTEX_WAKE2 */ + __umtx_op_sem2_wait, /* UMTX_OP_SEM2_WAIT */ + __umtx_op_sem2_wake, /* UMTX_OP_SEM2_WAKE */ }; int @@ -3320,6 +3468,7 @@ __umtx_op_wait_uint_private_compat32(str return do_wait(td, uap->obj, uap->val, tm_p, 1, 1); } +#if defined(COMPAT_FREEBSD9) || defined(COMPAT_FREEBSD10) static int __umtx_op_sem_wait_compat32(struct thread *td, struct _umtx_op_args *uap) { @@ -3338,6 +3487,26 @@ __umtx_op_sem_wait_compat32(struct threa } return (do_sem_wait(td, uap->obj, tm_p)); } +#endif + +static int +__umtx_op_sem2_wait_compat32(struct thread *td, struct _umtx_op_args *uap) +{ + struct _umtx_time *tm_p, timeout; + int error; + + /* Allow a null timespec (wait forever). */ + if (uap->uaddr2 == NULL) + tm_p = NULL; + else { + error = umtx_copyin_umtx_time32(uap->uaddr2, + (size_t)uap->uaddr1, &timeout); + if (error != 0) + return (error); + tm_p = &timeout; + } + return (do_sem2_wait(td, uap->obj, tm_p)); +} static int __umtx_op_nwake_private32(struct thread *td, struct _umtx_op_args *uap) @@ -3385,10 +3554,17 @@ static _umtx_op_func op_table_compat32[] __umtx_op_wake_private, /* UMTX_OP_WAKE_PRIVATE */ __umtx_op_wait_umutex_compat32, /* UMTX_OP_MUTEX_WAIT */ __umtx_op_wake_umutex, /* UMTX_OP_MUTEX_WAKE */ +#if defined(COMPAT_FREEBSD9) || defined(COMPAT_FREEBSD10) __umtx_op_sem_wait_compat32, /* UMTX_OP_SEM_WAIT */ __umtx_op_sem_wake, /* UMTX_OP_SEM_WAKE */ +#else + __umtx_op_unimpl, /* UMTX_OP_SEM_WAIT */ + __umtx_op_unimpl, /* UMTX_OP_SEM_WAKE */ +#endif __umtx_op_nwake_private32, /* UMTX_OP_NWAKE_PRIVATE */ - __umtx_op_wake2_umutex /* UMTX_OP_MUTEX_WAKE2 */ + __umtx_op_wake2_umutex, /* UMTX_OP_MUTEX_WAKE2 */ + __umtx_op_sem2_wait_compat32, /* UMTX_OP_SEM2_WAIT */ + __umtx_op_sem2_wake, /* UMTX_OP_SEM2_WAKE */ }; int Modified: head/sys/sys/_umtx.h ============================================================================== --- head/sys/sys/_umtx.h Fri Oct 24 19:58:24 2014 (r273603) +++ head/sys/sys/_umtx.h Fri Oct 24 20:02:44 2014 (r273604) @@ -61,6 +61,11 @@ struct _usem { __uint32_t _flags; }; +struct _usem2 { + volatile __uint32_t _count; /* Waiters flag in high bit. */ + __uint32_t _flags; +}; + struct _umtx_time { struct timespec _timeout; __uint32_t _flags; Modified: head/sys/sys/umtx.h ============================================================================== --- head/sys/sys/umtx.h Fri Oct 24 19:58:24 2014 (r273603) +++ head/sys/sys/umtx.h Fri Oct 24 20:02:44 2014 (r273604) @@ -52,6 +52,11 @@ /* _usem flags */ #define SEM_NAMED 0x0002 +/* _usem2 count field */ +#define USEM_HAS_WAITERS 0x80000000U +#define USEM_MAX_COUNT 0x7fffffffU +#define USEM_COUNT(c) ((c) & USEM_MAX_COUNT) + /* op code for _umtx_op */ #define UMTX_OP_RESERVED0 0 #define UMTX_OP_RESERVED1 1 @@ -72,11 +77,13 @@ #define UMTX_OP_WAKE_PRIVATE 16 #define UMTX_OP_MUTEX_WAIT 17 #define UMTX_OP_MUTEX_WAKE 18 /* deprecated */ -#define UMTX_OP_SEM_WAIT 19 -#define UMTX_OP_SEM_WAKE 20 +#define UMTX_OP_SEM_WAIT 19 /* deprecated */ +#define UMTX_OP_SEM_WAKE 20 /* deprecated */ #define UMTX_OP_NWAKE_PRIVATE 21 #define UMTX_OP_MUTEX_WAKE2 22 -#define UMTX_OP_MAX 23 +#define UMTX_OP_SEM2_WAIT 23 +#define UMTX_OP_SEM2_WAKE 24 +#define UMTX_OP_MAX 25 /* Flags for UMTX_OP_CV_WAIT */ #define CVWAIT_CHECK_UNPARKING 0x01 Modified: head/usr.bin/truss/syscalls.c ============================================================================== --- head/usr.bin/truss/syscalls.c Fri Oct 24 19:58:24 2014 (r273603) +++ head/usr.bin/truss/syscalls.c Fri Oct 24 20:02:44 2014 (r273604) @@ -424,6 +424,7 @@ static struct xlat umtx_ops[] = { X(UMTX_OP_WAIT_UINT_PRIVATE) X(UMTX_OP_WAKE_PRIVATE) X(UMTX_OP_MUTEX_WAIT) X(UMTX_OP_MUTEX_WAKE) X(UMTX_OP_SEM_WAIT) X(UMTX_OP_SEM_WAKE) X(UMTX_OP_NWAKE_PRIVATE) X(UMTX_OP_MUTEX_WAKE2) + X(UMTX_OP_SEM2_WAIT) X(UMTX_OP_SEM2_WAKE) XEND }; 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