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    SubjectRe: [PATCH] blk-mq: use mutex_trylock to avoid lock inversion
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    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
    >
    >
    >

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