+/*
+ * 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;
+
+ /* Lock just the semaphore we are interested in. */
+ spin_lock(&sem->lock);
+
+ /*
+ * If sma->complex_count was set while we were spinning,
+ * we may need to look at things we did not lock here.
+ */
+ if (unlikely(sma->complex_count)) {
+ spin_unlock(&sem->lock);
+ goto lock_array;
+ }
+
+ /*
+ * 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.
+ */
+ if (unlikely(spin_is_locked(&sma->sem_perm.lock))) {
+ spin_unlock(&sem->lock);
+ spin_unlock_wait(&sma->sem_perm.lock);
+ goto again;
+ }
+
+ locknum = 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.
+ */
+ lock_array:
+ spin_lock(&sma->sem_perm.lock);
+ for (i = 0; i < sma->sem_nsems; i++) {
+ struct sem *sem = sma->sem_base + i;
+ spin_unlock_wait(&sem->lock);
+ }
+ locknum = -1;
+ }
+ return locknum;
+}
+
+static inline void sem_unlock(struct sem_array *sma, int locknum)
+{
+ if (locknum == -1) {
+ spin_unlock(&sma->sem_perm.lock);
+ } else {
+ struct sem *sem = sma->sem_base + locknum;
+ spin_unlock(&sem->lock);
+ }
+}
+