ocfs2: Protect periodic quota syncing with s_umount semaphore

New quota locking rules will require s_umount semaphore for all quota
scanning functions. Add is for periodic quota syncing.
Tested-by: Eric Ren <zren@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/ocfs2/quota_global.c

@@ -634,7 +634,15 @@ static void qsync_work_fn(struct work_struct *work)
 						      dqi_sync_work.work);
 	struct super_block *sb = oinfo->dqi_gqinode->i_sb;
 
+	/*
+	 * We have to be careful here not to deadlock on s_umount as umount
+	 * disabling quotas may be in progress and it waits for this work to
+	 * complete. If trylock fails, we'll do the sync next time...
+	 */
+	if (down_read_trylock(&sb->s_umount)) {
 		dquot_scan_active(sb, ocfs2_sync_dquot_helper, oinfo->dqi_type);
+		up_read(&sb->s_umount);
+	}
 	schedule_delayed_work(&oinfo->dqi_sync_work,
 			      msecs_to_jiffies(oinfo->dqi_syncms));
 }

fs/ocfs2/super.c

@@ -985,7 +985,6 @@ static void ocfs2_disable_quotas(struct ocfs2_super *osb)
 	for (type = 0; type < OCFS2_MAXQUOTAS; type++) {
 		if (!sb_has_quota_loaded(sb, type))
 			continue;
-		/* Cancel periodic syncing before we grab dqonoff_mutex */
 		oinfo = sb_dqinfo(sb, type)->dqi_priv;
 		cancel_delayed_work_sync(&oinfo->dqi_sync_work);
 		inode = igrab(sb->s_dquot.files[type]);