bcache: fix race of writeback thread starting before complete initialization

Eric Wheeler · axboe · commit 07cc6ef8edc4 · 2016-03-08T09:17:30.000-07:00
The bch_writeback_thread might BUG_ON in read_dirty() if dc->sb==BDEV_STATE_DIRTY and bch_sectors_dirty_init has not yet completed its related initialization. This patch downs the dc->writeback_lock until after initialization is complete, thus preventing bch_writeback_thread from proceeding prematurely. See this thread: http://thread.gmane.org/gmane.linux.kernel.bcache.devel/3453 Signed-off-by: Eric Wheeler <bcache@linux.ewheeler.net> Tested-by: Marc MERLIN <marc@merlins.org> Cc: <stable@vger.kernel.org> Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
@@ -1015,8 +1015,12 @@ int bch_cached_dev_attach(struct cached_dev *dc, struct cache_set *c)
 	 */
 	atomic_set(&dc->count, 1);
 
-	if (bch_cached_dev_writeback_start(dc))
+	/* Block writeback thread, but spawn it */
+	down_write(&dc->writeback_lock);
+	if (bch_cached_dev_writeback_start(dc)) {
+		up_write(&dc->writeback_lock);
 		return -ENOMEM;
+	}
 
 	if (BDEV_STATE(&dc->sb) == BDEV_STATE_DIRTY) {
 		bch_sectors_dirty_init(dc);
@@ -1028,6 +1032,9 @@ int bch_cached_dev_attach(struct cached_dev *dc, struct cache_set *c)
 	bch_cached_dev_run(dc);
 	bcache_device_link(&dc->disk, c, "bdev");
 
+	/* Allow the writeback thread to proceed */
+	up_write(&dc->writeback_lock);
+
 	pr_info("Caching %s as %s on set %pU",
 		bdevname(dc->bdev, buf), dc->disk.disk->disk_name,
 		dc->disk.c->sb.set_uuid);
