diff options
Diffstat (limited to 'ipc/sem.c')
| -rw-r--r-- | ipc/sem.c | 180 |
1 files changed, 123 insertions, 57 deletions
@@ -253,70 +253,112 @@ static void sem_rcu_free(struct rcu_head *head) } /* + * Wait until all currently ongoing simple ops have completed. + * Caller must own sem_perm.lock. + * New simple ops cannot start, because simple ops first check + * that sem_perm.lock is free. + * that a) sem_perm.lock is free and b) complex_count is 0. + */ +static void sem_wait_array(struct sem_array *sma) +{ + int i; + struct sem *sem; + + if (sma->complex_count) { + /* The thread that increased sma->complex_count waited on + * all sem->lock locks. Thus we don't need to wait again. + */ + return; + } + + for (i = 0; i < sma->sem_nsems; i++) { + sem = sma->sem_base + i; + spin_unlock_wait(&sem->lock); + } +} + +/* * If the request contains only one semaphore operation, and there are * no complex transactions pending, lock only the semaphore involved. * Otherwise, lock the entire semaphore array, since we either have * multiple semaphores in our own semops, or we need to look at * semaphores from other pending complex operations. - * - * Carefully guard against sma->complex_count changing between zero - * and non-zero while we are spinning for the lock. The value of - * sma->complex_count cannot change while we are holding the lock, - * so sem_unlock should be fine. - * - * The global lock path checks that all the local locks have been released, - * checking each local lock once. This means that the local lock paths - * cannot start their critical sections while the global lock is held. */ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops, int nsops) { - int locknum; - again: - if (nsops == 1 && !sma->complex_count) { - struct sem *sem = sma->sem_base + sops->sem_num; + struct sem *sem; - /* Lock just the semaphore we are interested in. */ - spin_lock(&sem->lock); + if (nsops != 1) { + /* Complex operation - acquire a full lock */ + ipc_lock_object(&sma->sem_perm); - /* - * If sma->complex_count was set while we were spinning, - * we may need to look at things we did not lock here. + /* And wait until all simple ops that are processed + * right now have dropped their locks. */ - if (unlikely(sma->complex_count)) { - spin_unlock(&sem->lock); - goto lock_array; - } + sem_wait_array(sma); + return -1; + } + /* + * Only one semaphore affected - try to optimize locking. + * The rules are: + * - optimized locking is possible if no complex operation + * is either enqueued or processed right now. + * - The test for enqueued complex ops is simple: + * sma->complex_count != 0 + * - Testing for complex ops that are processed right now is + * a bit more difficult. Complex ops acquire the full lock + * and first wait that the running simple ops have completed. + * (see above) + * Thus: If we own a simple lock and the global lock is free + * and complex_count is now 0, then it will stay 0 and + * thus just locking sem->lock is sufficient. + */ + sem = sma->sem_base + sops->sem_num; + + if (sma->complex_count == 0) { /* - * Another process is holding the global lock on the - * sem_array; we cannot enter our critical section, - * but have to wait for the global lock to be released. + * It appears that no complex operation is around. + * Acquire the per-semaphore lock. */ - if (unlikely(spin_is_locked(&sma->sem_perm.lock))) { - spin_unlock(&sem->lock); - spin_unlock_wait(&sma->sem_perm.lock); - goto again; + spin_lock(&sem->lock); + + /* Then check that the global lock is free */ + if (!spin_is_locked(&sma->sem_perm.lock)) { + /* spin_is_locked() is not a memory barrier */ + smp_mb(); + + /* Now repeat the test of complex_count: + * It can't change anymore until we drop sem->lock. + * Thus: if is now 0, then it will stay 0. + */ + if (sma->complex_count == 0) { + /* fast path successful! */ + return sops->sem_num; + } } + spin_unlock(&sem->lock); + } - locknum = sops->sem_num; + /* slow path: acquire the full lock */ + ipc_lock_object(&sma->sem_perm); + + if (sma->complex_count == 0) { + /* False alarm: + * There is no complex operation, thus we can switch + * back to the fast path. + */ + spin_lock(&sem->lock); + ipc_unlock_object(&sma->sem_perm); + return sops->sem_num; } else { - int i; - /* - * Lock the semaphore array, and wait for all of the - * individual semaphore locks to go away. The code - * above ensures no new single-lock holders will enter - * their critical section while the array lock is held. + /* Not a false alarm, thus complete the sequence for a + * full lock. */ - lock_array: - ipc_lock_object(&sma->sem_perm); - for (i = 0; i < sma->sem_nsems; i++) { - struct sem *sem = sma->sem_base + i; - spin_unlock_wait(&sem->lock); - } - locknum = -1; + sem_wait_array(sma); + return -1; } - return locknum; } static inline void sem_unlock(struct sem_array *sma, int locknum) @@ -876,6 +918,24 @@ again: } /** + * set_semotime(sma, sops) - set sem_otime + * @sma: semaphore array + * @sops: operations that modified the array, may be NULL + * + * sem_otime is replicated to avoid cache line trashing. + * This function sets one instance to the current time. + */ +static void set_semotime(struct sem_array *sma, struct sembuf *sops) +{ + if (sops == NULL) { + sma->sem_base[0].sem_otime = get_seconds(); + } else { + sma->sem_base[sops[0].sem_num].sem_otime = + get_seconds(); + } +} + +/** * do_smart_update(sma, sops, nsops, otime, pt) - optimized update_queue * @sma: semaphore array * @sops: operations that were performed @@ -925,17 +985,10 @@ static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsop } } } - if (otime) { - if (sops == NULL) { - sma->sem_base[0].sem_otime = get_seconds(); - } else { - sma->sem_base[sops[0].sem_num].sem_otime = - get_seconds(); - } - } + if (otime) + set_semotime(sma, sops); } - /* The following counts are associated to each semaphore: * semncnt number of tasks waiting on semval being nonzero * semzcnt number of tasks waiting on semval being zero @@ -1797,12 +1850,17 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops, error = perform_atomic_semop(sma, sops, nsops, un, task_tgid_vnr(current)); - if (error <= 0) { - if (alter && error == 0) + if (error == 0) { + /* If the operation was successful, then do + * the required updates. + */ + if (alter) do_smart_update(sma, sops, nsops, 1, &tasks); - - goto out_unlock_free; + else + set_semotime(sma, sops); } + if (error <= 0) + goto out_unlock_free; /* We need to sleep on this operation, so we put the current * task into the pending queue and go to sleep. @@ -2061,6 +2119,14 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it) struct sem_array *sma = it; time_t sem_otime; + /* + * The proc interface isn't aware of sem_lock(), it calls + * ipc_lock_object() directly (in sysvipc_find_ipc). + * In order to stay compatible with sem_lock(), we must wait until + * all simple semop() calls have left their critical regions. + */ + sem_wait_array(sma); + sem_otime = get_semotime(sma); return seq_printf(s, |
