[PATCH] locks: don't unnecessarily fail posix lock operations

posix_lock_file() was too cautious, failing operations on OOM, even if they
didn't actually require an allocation.

This has the disadvantage, that a failing unlock on process exit could lead to
a memory leak.  There are two possibilites for this:

- filesystem implements .lock() and calls back to posix_lock_file().  On
cleanup of files_struct locks_remove_posix() is called which should remove all
locks belonging to files_struct.  However if filesystem calls
posix_lock_file() which fails, then those locks will never be freed.

- if a file is closed while a lock is blocked, then after acquiring
fcntl_setlk() will undo the lock.  But this unlock itself might fail on OOM,
again possibly leaking the lock.

The solution is to move the checking of the allocations until after it is sure
that they will be needed.  This will solve the above problem since unlock will
always succeed unless it splits an existing region.

Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/locks.c

@@ -834,14 +834,7 @@ static int __posix_lock_file_conf(struct inode *inode, struct file_lock *request
 	if (request->fl_flags & FL_ACCESS)
 		goto out;
 
-	error = -ENOLCK; /* "no luck" */
-	if (!(new_fl && new_fl2))
-		goto out;
-
 	/*
-	 * We've allocated the new locks in advance, so there are no
-	 * errors possible (and no blocking operations) from here on.
-	 * 
 	 * Find the first old lock with the same owner as the new lock.
 	 */
 	
@@ -938,10 +931,25 @@ static int __posix_lock_file_conf(struct inode *inode, struct file_lock *request
 		before = &fl->fl_next;
 	}
 
+	/*
+	 * The above code only modifies existing locks in case of
+	 * merging or replacing.  If new lock(s) need to be inserted
+	 * all modifications are done bellow this, so it's safe yet to
+	 * bail out.
+	 */
+	error = -ENOLCK; /* "no luck" */
+	if (right && left == right && !new_fl2)
+		goto out;
+
 	error = 0;
 	if (!added) {
 		if (request->fl_type == F_UNLCK)
 			goto out;
+
+		if (!new_fl) {
+			error = -ENOLCK;
+			goto out;
+		}
 		locks_copy_lock(new_fl, request);
 		locks_insert_lock(before, new_fl);
 		new_fl = NULL;