VFS: Allow caller to determine if BSD or posix locks were actually freed

Change posix_lock_file_conf(), and flock_lock_file() so that if called
with an F_UNLCK argument, and the FL_EXISTS flag they will indicate
whether or not any locks were actually freed by returning 0 or -ENOENT.

Signed-off-by: Trond Myklebust <[e-mail]>

fs/locks.c

···
 /* Try to create a FLOCK lock on filp. We always insert new FLOCK locks
  * at the head of the list, but that's secret knowledge known only to
  * flock_lock_file and posix_lock_file.
+ *
+ * Note that if called with an FL_EXISTS argument, the caller may determine
+ * whether or not a lock was successfully freed by testing the return
+ * value for -ENOENT.
  */
 static int flock_lock_file(struct file *filp, struct file_lock *request)
 {
···
 		break;
 	}
 
-	if (request->fl_type == F_UNLCK)
+	if (request->fl_type == F_UNLCK) {
+		if ((request->fl_flags & FL_EXISTS) && !found)
+			error = -ENOENT;
 		goto out;
+	}
 
 	error = -ENOMEM;
 	new_fl = locks_alloc_lock();
···
 
 	error = 0;
 	if (!added) {
-		if (request->fl_type == F_UNLCK)
+		if (request->fl_type == F_UNLCK) {
+			if (request->fl_flags & FL_EXISTS)
+				error = -ENOENT;
 			goto out;
+		}
 
 		if (!new_fl) {
 			error = -ENOLCK;
···
  * Add a POSIX style lock to a file.
  * We merge adjacent & overlapping locks whenever possible.
  * POSIX locks are sorted by owner task, then by starting address
+ *
+ * Note that if called with an FL_EXISTS argument, the caller may determine
+ * whether or not a lock was successfully freed by testing the return
+ * value for -ENOENT.
  */
 int posix_lock_file(struct file *filp, struct file_lock *fl)
 {

include/linux/fs.h

···
 #define FL_POSIX	1
 #define FL_FLOCK	2
 #define FL_ACCESS	8	/* not trying to lock, just looking */
+#define FL_EXISTS	16	/* when unlocking, test for existence */
 #define FL_LEASE	32	/* lease held on this file */
 #define FL_CLOSE	64	/* unlock on close */
 #define FL_SLEEP	128	/* A blocking lock */