[PATCH v2 2/2] xfs: disable log force in xfs_buf_item_push() to avoid xa_lock recursion

[Brian Foster]

If the trylock fails and the buffer is pinned and stale, the
resulting xfs_log_force() can lead to lock recursion on the ailp
xa_lock. Call __xfs_buf_trylock() to attempt the buf lock with
xa_lock held, but to avoid the log force. If the trylock had raced
with the buffer being pinned, return XFS_ITEM_PINNED such that
xfsaild will pend up a log force on the next scan.

Signed-off-by: Brian Foster <bfoster at redhat.com>
---
 fs/xfs/xfs_buf_item.c |   14 +++++++++++++-
 1 files changed, 13 insertions(+), 1 deletions(-)

diff --git a/fs/xfs/xfs_buf_item.c b/fs/xfs/xfs_buf_item.c
index 9c4c050..23a328e 100644
--- a/fs/xfs/xfs_buf_item.c
+++ b/fs/xfs/xfs_buf_item.c
@@ -469,8 +469,20 @@ xfs_buf_item_push(
 
 	if (xfs_buf_ispinned(bp))
 		return XFS_ITEM_PINNED;
-	if (!xfs_buf_trylock(bp))
+
+	if (!__xfs_buf_trylock(bp, false)) {
+		/*
+		 * We disable the log force via trylock because the buffer can
+		 * become pinned and stale after the trylock fails and the log
+		 * force is unsafe with xa_lock held.
+		 *
+		 * Check the buffer state once more and return pinned such that
+		 * xfsaild pends up the log force when it drops xa_lock.
+		 */
+		if (xfs_buf_ispinned(bp))
+			return XFS_ITEM_PINNED;
 		return XFS_ITEM_LOCKED;
+	}
 
 	ASSERT(!(bip->bli_flags & XFS_BLI_STALE));
 
-- 
1.7.7.6