blkmq: Fix NULL pointer deref when all reserved tags in use

When allocating from the reserved tags pool, bt_get() is called with a NULL hctx. If all tags are in use, the hw queue is kicked to push out any pending IO, potentially freeing tags, and tag allocation is retried. The problem is that blk_mq_run_hw_queue() doesn't check for a NULL hctx. So we avoid it with a simple NULL hctx test.

This issue was introduced by:
b32232073e80: blk-mq: fix hang in bt_get()

Tested by hammering mtip32xx with concurrent smartctl/hdparm.

---

index d53a764..9d7dd64 100644
@@ -280,7 +280,8 @@ static int bt_get(struct blk_mq_alloc_data *data,
 		 * pending IO submits before going to sleep waiting for
 		 * some to complete.
 		 */
-		blk_mq_run_hw_queue(hctx, false);
+		if (hctx)
+			blk_mq_run_hw_queue(hctx, false);
 
 		/*
 		 * Retry tag allocation after running the hardware queue,

Thanks Sam, added to the current series.

Jens Axboe