xfs: cancel eofblocks background trimming on remount read-only

The filesystem quiesce sequence performs the operations necessary to
drain all background work, push pending transactions through the log
infrastructure and wait on I/O resulting from the final AIL push. We
have had reports of remount,ro hangs in xfs_log_quiesce() ->
xfs_wait_buftarg(), however, and some instrumentation code to detect
transaction commits at this point in the quiesce sequence has inculpated
the eofblocks background scanner as a cause.

While higher level remount code generally prevents user modifications by
the time the filesystem has made it to xfs_log_quiesce(), the background
scanner may still be alive and can perform pending work at any time. If
this occurs between the xfs_log_force() and xfs_wait_buftarg() calls
within xfs_log_quiesce(), this can lead to an indefinite lockup in
xfs_wait_buftarg().

To prevent this problem, cancel the background eofblocks scan worker
during the remount read-only quiesce sequence. This suspends background
trimming when a filesystem is remounted read-only. This is only done in
the remount path because the freeze codepath has already locked out new
transactions by the time the filesystem attempts to quiesce (and thus
waiting on an active work item could deadlock). Kick the eofblocks
worker to pick up where it left off once an fs is remounted back to
read-write.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_icache.c

@@ -765,7 +765,7 @@ restart:
  * Background scanning to trim post-EOF preallocated space. This is queued
  * based on the 'speculative_prealloc_lifetime' tunable (5m by default).
  */
-STATIC void
+void
 xfs_queue_eofblocks(
 	struct xfs_mount *mp)
 {

fs/xfs/xfs_icache.h

@@ -68,6 +68,7 @@ void xfs_inode_clear_eofblocks_tag(struct xfs_inode *ip);
 int xfs_icache_free_eofblocks(struct xfs_mount *, struct xfs_eofblocks *);
 int xfs_inode_free_quota_eofblocks(struct xfs_inode *ip);
 void xfs_eofblocks_worker(struct work_struct *);
+void xfs_queue_eofblocks(struct xfs_mount *);
 
 int xfs_inode_ag_iterator(struct xfs_mount *mp,
 	int (*execute)(struct xfs_inode *ip, int flags, void *args),

fs/xfs/xfs_super.c

@@ -1294,6 +1294,7 @@ xfs_fs_remount(
 		 */
 		xfs_restore_resvblks(mp);
 		xfs_log_work_queue(mp);
+		xfs_queue_eofblocks(mp);
 	}
 
 	/* rw -> ro */
@@ -1306,6 +1307,13 @@ xfs_fs_remount(
 		 * return it to the same size.
 		 */
 		xfs_save_resvblks(mp);
+
+		/*
+		 * Cancel background eofb scanning so it cannot race with the
+		 * final log force+buftarg wait and deadlock the remount.
+		 */
+		cancel_delayed_work_sync(&mp->m_eofblocks_work);
+
 		xfs_quiesce_attr(mp);
 		mp->m_flags |= XFS_MOUNT_RDONLY;
 	}