[PATCH v3 3/4] gfs2: protect quota sync sysfs callback

[Jiacheng Xu]

The quota_sync sysfs callback can run while quota cleanup is releasing
the quota bitmap and internal quota inodes during mount rollback or
unmount.

Hold s_umount for reading while synchronizing quota data and reject
requests before the superblock is active. Use trylock semantics so the
callback cannot deadlock with the write-side teardown path.

Signed-off-by: Jiacheng Xu <stitch@xxxxxxxxxx>
---
 fs/gfs2/sys.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/gfs2/sys.c b/fs/gfs2/sys.c
index fd6e8e34651d..c5868354073a 100644
--- a/fs/gfs2/sys.c
+++ b/fs/gfs2/sys.c
@@ -254,6 +254,7 @@ static ssize_t statfs_sync_store(struct gfs2_sbd *sdp, const char *buf,
 static ssize_t quota_sync_store(struct gfs2_sbd *sdp, const char *buf,
 				size_t len)
 {
+	struct super_block *sb = sdp->sd_vfs;
 	int error, val;
 
 	if (!capable(CAP_SYS_ADMIN))
@@ -266,7 +267,12 @@ static ssize_t quota_sync_store(struct gfs2_sbd *sdp, const char *buf,
 	if (val != 1)
 		return -EINVAL;
 
-	gfs2_quota_sync(sdp->sd_vfs, 0);
+	if (!super_trylock_shared_active(sb))
+		return -EAGAIN;
+
+	gfs2_quota_sync(sb, 0);
+
+	super_unlock_active(sb);
 	return len;
 }
 
-- 
2.51.0