GFS2: Protect quota sync generation

Now that gfs2_quota_sync can be potentially called from multiple
threads, we should protect this bit of code, and the sync generation
number in particular in order to ensure that there are no races
when syncing quotas.
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Cc: Abhijith Das <adas@redhat.com>

fs/gfs2/incore.h

@@ -714,6 +714,7 @@ struct gfs2_sbd {
 	struct list_head sd_quota_list;
 	atomic_t sd_quota_count;
 	struct mutex sd_quota_mutex;
+	struct mutex sd_quota_sync_mutex;
 	wait_queue_head_t sd_quota_wait;
 	struct list_head sd_trunc_list;
 	spinlock_t sd_trunc_lock;

fs/gfs2/ops_fstype.c

@@ -93,6 +93,7 @@ static struct gfs2_sbd *init_sbd(struct super_block *sb)
 
 	INIT_LIST_HEAD(&sdp->sd_quota_list);
 	mutex_init(&sdp->sd_quota_mutex);
+	mutex_init(&sdp->sd_quota_sync_mutex);
 	init_waitqueue_head(&sdp->sd_quota_wait);
 	INIT_LIST_HEAD(&sdp->sd_trunc_list);
 	spin_lock_init(&sdp->sd_trunc_lock);

fs/gfs2/quota.c

@@ -1132,12 +1132,13 @@ int gfs2_quota_sync(struct super_block *sb, int type)
 	unsigned int x;
 	int error = 0;
 
-	sdp->sd_quota_sync_gen++;
-
 	qda = kcalloc(max_qd, sizeof(struct gfs2_quota_data *), GFP_KERNEL);
 	if (!qda)
 		return -ENOMEM;
 
+	mutex_lock(&sdp->sd_quota_sync_mutex);
+	sdp->sd_quota_sync_gen++;
+
 	do {
 		num_qd = 0;
 
@@ -1162,6 +1163,7 @@ int gfs2_quota_sync(struct super_block *sb, int type)
 		}
 	} while (!error && num_qd == max_qd);
 
+	mutex_unlock(&sdp->sd_quota_sync_mutex);
 	kfree(qda);
 
 	return error;