linux-kernel - Re: [PATCH] block: Fix warning when I/O elevator is changed as request

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Wed, 23 Aug 2017 11:34:40 +0800
From:   Ming Lei <tom.leiming@...il.com>
To:     David Jeffery <djeffery@...hat.com>
Cc:     linux-block <linux-block@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] block: Fix warning when I/O elevator is changed as
 request_queue is being removed

On Wed, Aug 9, 2017 at 9:44 AM, Ming Lei <tom.leiming@...il.com> wrote:
> Hi David,
>
> On Wed, Aug 9, 2017 at 2:13 AM, David Jeffery <djeffery@...hat.com> wrote:
>> On 08/07/2017 07:53 PM, Ming Lei wrote:
>>> On Tue, Aug 8, 2017 at 3:38 AM, David Jeffery <djeffery@...hat.com> wrote:
>>
>>>>
>>>> Signed-off-by: David Jeffery <djeffery@...hat.com>
>>>> ---
>>>>  block/blk-sysfs.c |    2 ++
>>>>  block/elevator.c  |    4 ++++
>>>>  2 files changed, 6 insertions(+)
>>>>
>>>>
>>>> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
>>>> index 27aceab..b8362c0 100644
>>>> --- a/block/blk-sysfs.c
>>>> +++ b/block/blk-sysfs.c
>>>> @@ -931,7 +931,9 @@ void blk_unregister_queue(struct gendisk *disk)
>>>>         if (WARN_ON(!q))
>>>>                 return;
>>>>
>>>> +       mutex_lock(&q->sysfs_lock);
>>>>         queue_flag_clear_unlocked(QUEUE_FLAG_REGISTERED, q);
>>>> +       mutex_unlock(&q->sysfs_lock);
>>>
>>> Could you share why the lock of 'q->sysfs_lock' is needed here?
>>
>> As the elevator change is initiated through a sysfs attr file in the
>> queue directory, the task doing the elevator change already acquires the
>> q->sysfs_lock before it can try and change the elevator.  Adding the
>
> It is e->sysfs_lock which is held in elv_attr_store(), instead of q->sysfs_lock.

Looks I was wrong, and the store is from queue_attr_store() instead of
elv_attr_store().

I can reproduce the issue and this patch can fix the issue in my test
on scsi_debug,
so:

        Tested-by: Ming Lei <ming.lei@...hat.com>

And it is a typical race between removing queue kobj and adding children of
this queue kobj, we can't acquire q->sysfs_lock in blk_unregister_queue()
because of sysfs's limit(may cause deadlock), so one state has to be used
for this purpose, just like what register/unregister hctx kobjs does,
and it should
be fine to use QUEUE_FLAG_REGISTERED here. More changes are required if
we use e->registered, so this patch looks fine:

       Reviewed-by: Ming Lei <ming.lei@...hat.com>


Thanks,
Ming Lei