[PATCH] xfs: acquire superblock freeze protection on eofblocks scans

[Brian Foster <bfoster@redhat.com>]

The filesystem freeze sequence in XFS waits on any background
eofblocks or cowblocks scans to complete before the filesystem is
quiesced. At this point, the freezer has already stopped the
transaction subsystem, however, which means a truncate or cowblock
cancellation in progress is likely blocked in transaction
allocation. This results in a deadlock between freeze and the
associated scanner.

Fix this problem by holding superblock write protection across calls
into the block reapers. Since protection for background scans is
acquired from the workqueue task context, trylock to avoid a similar
deadlock between freeze and blocking on the write lock.

Fixes: d6b636ebb1c9f ("xfs: halt auto-reclamation activities while rebuilding rmap")
Reported-by: Paul Furtado <paulfurtado91@gmail.com>
Signed-off-by: Brian Foster <bfoster@redhat.com>
---

Note that this has the opposite tradeoff as the approach I originally
posited [1], specifically that the eofblocks ioctl() now always blocks
on a frozen fs rather than return -EAGAIN. It's worth pointing out that
the eofb control structure has a sync flag (that is not used for
background scans), so yet another approach could be to tie the trylock
to that.

Brian

[1] https://lore.kernel.org/linux-xfs/20200407163739.GG28936@bfoster/

 fs/xfs/xfs_icache.c | 10 ++++++++++
 fs/xfs/xfs_ioctl.c  |  5 ++++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
index a7be7a9e5c1a..8bf1d15be3f6 100644
--- a/fs/xfs/xfs_icache.c
+++ b/fs/xfs/xfs_icache.c
@@ -911,7 +911,12 @@ xfs_eofblocks_worker(
 {
 	struct xfs_mount *mp = container_of(to_delayed_work(work),
 				struct xfs_mount, m_eofblocks_work);
+
+	if (!sb_start_write_trylock(mp->m_super))
+		return;
 	xfs_icache_free_eofblocks(mp, NULL);
+	sb_end_write(mp->m_super);
+
 	xfs_queue_eofblocks(mp);
 }
 
@@ -938,7 +943,12 @@ xfs_cowblocks_worker(
 {
 	struct xfs_mount *mp = container_of(to_delayed_work(work),
 				struct xfs_mount, m_cowblocks_work);
+
+	if (!sb_start_write_trylock(mp->m_super))
+		return;
 	xfs_icache_free_cowblocks(mp, NULL);
+	sb_end_write(mp->m_super);
+
 	xfs_queue_cowblocks(mp);
 }
 
diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
index cdfb3cd9a25b..309958186d33 100644
--- a/fs/xfs/xfs_ioctl.c
+++ b/fs/xfs/xfs_ioctl.c
@@ -2363,7 +2363,10 @@ xfs_file_ioctl(
 		if (error)
 			return error;
 
-		return xfs_icache_free_eofblocks(mp, &keofb);
+		sb_start_write(mp->m_super);
+		error = xfs_icache_free_eofblocks(mp, &keofb);
+		sb_end_write(mp->m_super);
+		return error;
 	}
 
 	default:
-- 
2.21.1