[PATCH 3.16 042/148] quota: fix livelock in dquot_writeback_dquots

[Ben Hutchings]

3.16.82-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Dmitry Monakhov <dmtrmonakhov@xxxxxxxxxxxxxx>

commit 6ff33d99fc5c96797103b48b7b0902c296f09c05 upstream.

Write only quotas which are dirty at entry.

XFSTEST: https://github.com/dmonakhov/xfstests/commit/b10ad23566a5bf75832a6f500e1236084083cddc

Link: https://lore.kernel.org/r/20191031103920.3919-1-dmonakhov@xxxxxxxxxx
Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Signed-off-by: Dmitry Monakhov <dmtrmonakhov@xxxxxxxxxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/quota/dquot.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

--- a/fs/quota/dquot.c
+++ b/fs/quota/dquot.c
@@ -607,7 +607,7 @@ EXPORT_SYMBOL(dquot_scan_active);
 /* Write all dquot structures to quota files */
 int dquot_writeback_dquots(struct super_block *sb, int type)
 {
-	struct list_head *dirty;
+	struct list_head dirty;
 	struct dquot *dquot;
 	struct quota_info *dqopt = sb_dqopt(sb);
 	int cnt;
@@ -620,9 +620,10 @@ int dquot_writeback_dquots(struct super_
 		if (!sb_has_quota_active(sb, cnt))
 			continue;
 		spin_lock(&dq_list_lock);
-		dirty = &dqopt->info[cnt].dqi_dirty_list;
-		while (!list_empty(dirty)) {
-			dquot = list_first_entry(dirty, struct dquot,
+		/* Move list away to avoid livelock. */
+		list_replace_init(&dqopt->info[cnt].dqi_dirty_list, &dirty);
+		while (!list_empty(&dirty)) {
+			dquot = list_first_entry(&dirty, struct dquot,
 						 dq_dirty);
 			/* Dirty and inactive can be only bad dquot... */
 			if (!test_bit(DQ_ACTIVE_B, &dquot->dq_flags)) {