Re: [PATCH] block: Fix warning when I/O elevator is changed as request_queue is being removed

[Jens Axboe]

On 08/27/2017 07:36 PM, Ming Lei wrote:
> On Wed, Aug 23, 2017 at 11:34 AM, Ming Lei <tom.leiming@xxxxxxxxx> wrote:
>> On Wed, Aug 9, 2017 at 9:44 AM, Ming Lei <tom.leiming@xxxxxxxxx> wrote:
>>> Hi David,
>>>
>>> On Wed, Aug 9, 2017 at 2:13 AM, David Jeffery <djeffery@xxxxxxxxxx> wrote:
>>>> On 08/07/2017 07:53 PM, Ming Lei wrote:
>>>>> On Tue, Aug 8, 2017 at 3:38 AM, David Jeffery <djeffery@xxxxxxxxxx> wrote:
>>>>
>>>>>>
>>>>>> Signed-off-by: David Jeffery <djeffery@xxxxxxxxxx>
>>>>>> ---
>>>>>>  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@xxxxxxxxxx>
>>
>> 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@xxxxxxxxxx>
> 
> 
> Hi Jens,
> 
> Could you consider this patch for v4.13 or v4.14?

Yep, added for 4.14, thanks.

-- 
Jens Axboe