[PATCH 11/11] xfs: avoid ABBA deadlock when scrubbing parent pointers

["Darrick J. Wong" <darrick.wong@oracle.com>]

From: Darrick J. Wong <darrick.wong@oracle.com>

In normal operation, the XFS convention is to take an inode's iolock
and then allocate a transaction.  However, when scrubbing parent inodes
this is inverted -- we allocated the transaction to do the scrub, and
now we're trying to grab the parent's iolock.  This can lead to ABBA
deadlocks: some thread grabbed the parent's iolock and is waiting for
space for a transaction while our parent scrubber is sitting on a
transaction trying to get the parent's iolock.

Therefore, convert all iolock attempts to use trylock; if that fails,
they can use the existing mechanisms to back off and try again.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
 fs/xfs/scrub/common.c |   22 ++++++++++++++++++++++
 fs/xfs/scrub/common.h |    2 ++
 fs/xfs/scrub/parent.c |   16 ++++++++++++++--
 3 files changed, 38 insertions(+), 2 deletions(-)


diff --git a/fs/xfs/scrub/common.c b/fs/xfs/scrub/common.c
index f5e281a..93f9e7d 100644
--- a/fs/xfs/scrub/common.c
+++ b/fs/xfs/scrub/common.c
@@ -787,3 +787,25 @@ xfs_scrub_buffer_recheck(
 	sc->sm->sm_flags |= XFS_SCRUB_OFLAG_CORRUPT;
 	trace_xfs_scrub_block_error(sc, bp->b_bn, fa);
 }
+
+/*
+ * 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
+xfs_scrub_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;
+}
diff --git a/fs/xfs/scrub/common.h b/fs/xfs/scrub/common.h
index 8296873..191c369 100644
--- a/fs/xfs/scrub/common.h
+++ b/fs/xfs/scrub/common.h
@@ -151,4 +151,6 @@ static inline bool xfs_scrub_found_corruption(struct xfs_scrub_metadata *sm)
 			       XFS_SCRUB_OFLAG_XCORRUPT);
 }
 
+int xfs_scrub_ilock_inverted(struct xfs_inode *ip, uint lock_mode);
+
 #endif	/* __XFS_SCRUB_COMMON_H__ */
diff --git a/fs/xfs/scrub/parent.c b/fs/xfs/scrub/parent.c
index 1fb88c1..19cd54d 100644
--- a/fs/xfs/scrub/parent.c
+++ b/fs/xfs/scrub/parent.c
@@ -211,7 +211,9 @@ xfs_scrub_parent_validate(
 	 */
 	xfs_iunlock(sc->ip, sc->ilock_flags);
 	sc->ilock_flags = 0;
-	xfs_ilock(dp, XFS_IOLOCK_SHARED);
+	error = xfs_scrub_ilock_inverted(dp, XFS_IOLOCK_SHARED);
+	if (error)
+		goto out_rele;
 
 	/* Go looking for our dentry. */
 	error = xfs_scrub_parent_count_parent_dentries(sc, dp, &nlink);
@@ -220,8 +222,10 @@ xfs_scrub_parent_validate(
 
 	/* Drop the parent lock, relock this inode. */
 	xfs_iunlock(dp, XFS_IOLOCK_SHARED);
+	error = xfs_scrub_ilock_inverted(sc->ip, XFS_IOLOCK_EXCL);
+	if (error)
+		goto out_rele;
 	sc->ilock_flags = XFS_IOLOCK_EXCL;
-	xfs_ilock(sc->ip, sc->ilock_flags);
 
 	/*
 	 * If we're an unlinked directory, the parent /won't/ have a link
@@ -323,5 +327,13 @@ xfs_scrub_parent(
 	if (try_again && tries == 20)
 		xfs_scrub_set_incomplete(sc);
 out:
+	/*
+	 * If we failed to lock the parent inode even after a retry, just mark
+	 * this scrub incomplete and return.
+	 */
+	if (sc->try_harder && error == -EDEADLOCK) {
+		error = 0;
+		xfs_scrub_set_incomplete(sc);
+	}
 	return error;
 }