Commit abab13b5 authored Oct 07, 2014 by Jens Axboe

blk-mq: fix potential hang if rolling wakeup depth is too high



We currently divide the queue depth by 4 as our batch wakeup
count, but we split the wakeups over BT_WAIT_QUEUES number of
wait queues. This defaults to 8. If the product of the resulting
batch wake count and BT_WAIT_QUEUES is higher than the device
queue depth, we can get into a situation where a task goes to
sleep waiting for a request, but never gets woken up.

Reported-by: Bart Van Assche <bvanassche@acm.org>
Fixes: 4bb659b1
Cc: stable@kernel.org
Signed-off-by: Jens Axboe <axboe@fb.com>

parent d8f429e1

block/blk-mq-tag.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -455,8 +455,8 @@ static void bt_update_count(struct blk_mq_bitmap_tags *bt,
		}

		bt->wake_cnt = BT_WAIT_BATCH;
		if (bt->wake_cnt > depth / 4)
		bt->wake_cnt = max(1U, depth / 4);
		if (bt->wake_cnt > depth / BT_WAIT_QUEUES)
		bt->wake_cnt = max(1U, depth / BT_WAIT_QUEUES);

		bt->depth = depth;
		}