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

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Date:   Thu, 21 Jun 2018 16:23:58 +0800
From:   "jianchao.wang" <jianchao.w.wang@...cle.com>
To:     Bart Van Assche <Bart.VanAssche@....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

Hi Bart

On 06/21/2018 12:18 AM, Bart Van Assche wrote:
> 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://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_osandov_blktests&d=DwIGaQ&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=7WdAxUBeiTUTCy8v-7zXyr4qk7sx26ATvfo6QSTvZyQ&m=_TjwODcX-0o-OZfrVTFMpvbS0QbYxoya17aHM9pZAXw&s=wdVpTkE6PJX3GXvjXNwEup3opdflyCGY324CAYXSixY&e=).
> * 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.
> 

Really appreciate your kindly and detailed directive.
I will post the V2 version based on your suggestions later.

Thanks
Jianchao
> 
> 
>