linux-kernel - Re: [PATCH] blk-mq: use mutex_trylock to avoid lock inversion

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Date:   Wed, 20 Jun 2018 16:18:49 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "jianchao.w.wang@...cle.com" <jianchao.w.wang@...cle.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>
Subject: Re: [PATCH] blk-mq: use mutex_trylock to avoid lock inversion

On Wed, 2018-06-20 at 10:09 +0800, jianchao.wang wrote:
> It is very easy to reproduce with following scripts.
> 
> script 0
> while true
> do
> 	modprobe null_blk queue_mode=2 shared_tags=1
> 	sleep 0.1
> 	rmmod null_blk
> 	sleep 0.1
> done
> 
> script 1
> file0="/sys/block/nullb0/mq/0/nr_tags"
> file1="/sys/block/nullb0/mq/0/cpu0/rq_list"
> while true;
> do
> 	if [ -e $file0 ];then
> 		cat $file0
> 	fi
> 	if [ -e $file1 ];then
> 		cat $file1
> 	fi
> done

Hello Jianchao,

Thanks for having shared a reproducer. However, the approach of the patch you
posted doesn't seem like the right approach to me. I propose to proceed as
follows:
* Convert the reproducer into a blktests test and submit is as a patch to the
  blktests project (https://github.com/osandov/blktests).
* Remove the mutex_lock/unlock(&sysfs_lock) calls from blk_cleanup_queue().
  These calls are useless.  Block drivers are required to call del_gendisk()
  before calling blk_cleanup_queue(). That means that sysfs attributes are
  removed synchronously before blk_cleanup_queue() is called. The following
  statement in blk_cleanup_queue() verifies that:
    WARN_ON_ONCE(q->kobj.state_in_sysfs);
  BTW, this also means that the blk_queue_dying() checks in various show and
  store methods are superfluous.
* Introduce a new mutex to serialize blk_mq_register_dev() and
  blk_mq_sysfs_register() and blk_mq_sysfs_unregister(). I think it is wrong
  that these functions use sysfs_mutex.
* Document the purpose of sysfs_mutex in include/linux/blkdev.h, namely to
  serialize the sysfs .show() and .store() callback functions and also to
  serialize elevator changes.

Thanks,

Bart.