ipc_rcu_free(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)
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,
projects / cascardo / linux.git / blobdiff