staging: lustre: ldlm: Fix a race during FLock handling

Protect against race where lock could have been just destroyed
due to overlap, in ldlm_process_flock_lock().
Easy reproducer is BULL's NFS Locktests in pthread mode.
(http://nfsv4.bullopensource.org/tools/tests/locktest.php)
Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-1126
Reviewed-on: http://review.whamcloud.com/7134Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/ldlm/ldlm_flock.c

@@ -520,11 +520,6 @@ ldlm_flock_completion_ast(struct ldlm_lock *lock, __u64 flags, void *data)
 granted:
 	OBD_FAIL_TIMEOUT(OBD_FAIL_LDLM_CP_CB_WAIT, 10);
 
-	if (lock->l_flags & LDLM_FL_DESTROYED) {
-		LDLM_DEBUG(lock, "client-side enqueue waking up: destroyed");
-		return 0;
-	}
-
 	if (lock->l_flags & LDLM_FL_FAILED) {
 		LDLM_DEBUG(lock, "client-side enqueue waking up: failed");
 		return -EIO;
@@ -534,6 +529,16 @@ ldlm_flock_completion_ast(struct ldlm_lock *lock, __u64 flags, void *data)
 
 	lock_res_and_lock(lock);
 
+	/*
+	 * Protect against race where lock could have been just destroyed
+	 * due to overlap in ldlm_process_flock_lock().
+	 */
+	if (lock->l_flags & LDLM_FL_DESTROYED) {
+		unlock_res_and_lock(lock);
+		LDLM_DEBUG(lock, "client-side enqueue waking up: destroyed");
+		return 0;
+	}
+
 	/* ldlm_lock_enqueue() has already placed lock on the granted list. */
 	list_del_init(&lock->l_res_link);