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

[Ming Lei]

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>


Thanks,
Ming Lei