Re: [PATCH V2] SCSI: fix queue cleanup race before queue initialization is done

On 11/14/18 1:25 AM, Ming Lei wrote:

c2856ae2f315d ("blk-mq: quiesce queue before freeing queue") has
already fixed this race, however the implied synchronize_rcu()
in blk_mq_quiesce_queue() can slow down LUN probe a lot, so caused
performance regression.

Then 1311326cf4755c7 ("blk-mq: avoid to synchronize rcu inside blk_cleanup_queue()")
tried to quiesce queue for avoiding unnecessary synchronize_rcu()
only when queue initialization is done, because it is usual to see
lots of inexistent LUNs which need to be probed.

However, turns out it isn't safe to quiesce queue only when queue
initialization is done. Because when one SCSI command is completed,
the user of sending command can be waken up immediately, then the
scsi device may be removed, meantime the run queue in scsi_end_request()
is still in-progress, so kernel panic can be caused.

In Red Hat QE lab, there are several reports about this kind of kernel
panic triggered during kernel booting.

This patch tries to address the issue by grabing one queue usage
counter during freeing one request and the following run queue.

Thanks applied, this bug was elusive but ever present in recent
testing that we did internally, it's been a huge pain in the butt.
The symptoms were usually a crash in blk_mq_get_driver_tag() with
hctx->tags == NULL, or a crash inside deadline request insert off
requeue.

-- 
Jens Axboe