[PATCH 07/10] xfs: prevent fs freeze with outstanding relog items

[Brian Foster <bfoster@redhat.com>]

The automatic relog mechanism is currently incompatible with
filesystem freeze in a generic sense. Freeze protection is currently
implemented via locks that cannot be held on return to userspace,
which means we can't hold a superblock write reference for the
duration relogging is enabled on an item. It's too late to block
when the freeze sequence calls into the filesystem because the
transaction subsystem has already begun to be frozen. Not only can
this block the relog transaction, but blocking any unrelated
transaction essentially prevents a particular operation from
progressing to the point where it can disable relogging on an item.
Therefore marking the relog transaction as "nowrite" does not solve
the problem.

This is not a problem in practice because the two primary use cases
already exclude freeze via other means. quotaoff holds ->s_umount
read locked across the operation and scrub explicitly takes a
superblock write reference, both of which block freeze of the
transaction subsystem for the duration of relog enabled items.

As a fallback for future use cases and the upcoming random buffer
relogging test code, fail fs freeze attempts when the global relog
reservation counter is elevated to prevent deadlock. This is a
partial punt of the problem as compared to teaching freeze to wait
on relogged items because the only current dependency is test code.
In other words, this patch prevents deadlock if a user happens to
issue a freeze in conjunction with random buffer relog injection.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
---
 fs/xfs/xfs_super.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 379cbff438bc..f77af5298a80 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -35,6 +35,7 @@
 #include "xfs_refcount_item.h"
 #include "xfs_bmap_item.h"
 #include "xfs_reflink.h"
+#include "xfs_trans_priv.h"
 
 #include <linux/magic.h>
 #include <linux/fs_context.h>
@@ -914,6 +915,9 @@ xfs_fs_freeze(
 {
 	struct xfs_mount	*mp = XFS_M(sb);
 
+	if (WARN_ON_ONCE(atomic64_read(&mp->m_ail->ail_relog_res)))
+		return -EAGAIN;
+
 	xfs_stop_block_reaping(mp);
 	xfs_save_resvblks(mp);
 	xfs_quiesce_attr(mp);
-- 
2.21.3