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: 4bb659b156996
Cc: stable@kernel.org
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index b087880..146fd02 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -455,8 +455,8 @@
}
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;
}