xfs: fix parent pointer scrub racing with subdirectory reparenting

Jan Kara pointed out that rename() doesn't lock a subdirectory that is
being moved from one parent to another, even though the move requires an
update to the subdirectory's dotdot entry.  This means that it's *not*
sufficient to hold a directory's IOLOCK to stabilize the dotdot entry.
We must hold the ILOCK of both the child and the alleged parent, and
there's no use in holding the parent's IOLOCK.

With that in mind, we can get rid of all the messy code that tries to
grab the parent's IOLOCK, which means we don't need to let go of the
ILOCK of the directory whose parent we are checking.  We still have to
use nonblocking mode to take the ILOCK of the alleged parent, so the
revalidation loop has to stay.

However, we can remove the retry counter, since threads aren't supposed
to hold the ILOCK for long periods of time.  Remove the inverted ilock
helper from the common code since nobody uses it.  Remove the entire
source of -EDEADLOCK-based "retry harder" scrub executions.

Link: https://lore.kernel.org/linux-xfs/20230117123735.un7wbamlbdihninm@quack3/
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

1 files changed, 0 insertions, 22 deletions

diff --git a/fs/xfs/scrub/common.c b/fs/xfs/scrub/common.c
index dcfe66044d4a..813ded91661b 100644
--- a/fs/xfs/scrub/common.c
+++ b/fs/xfs/scrub/common.c
@@ -962,28 +962,6 @@ xchk_metadata_inode_forks(
 	return 0;
 }
 
-/*
- * Try to lock an inode in violation of the usual locking order rules.  For
- * example, trying to get the IOLOCK while in transaction context, or just
- * plain breaking AG-order or inode-order inode locking rules.  Either way,
- * the only way to avoid an ABBA deadlock is to use trylock and back off if
- * we can't.
- */
-int
-xchk_ilock_inverted(
-	struct xfs_inode	*ip,
-	uint			lock_mode)
-{
-	int			i;
-
-	for (i = 0; i < 20; i++) {
-		if (xfs_ilock_nowait(ip, lock_mode))
-			return 0;
-		delay(1);
-	}
-	return -EDEADLOCK;
-}
-
 /* Pause background reaping of resources. */
 void
 xchk_stop_reaping(