Re: [PATCH v8 13/20] jobs: group together API calls under the same job lock

[Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>]

On 7/5/22 16:01, Emanuele Giuseppe Esposito wrote:
> 
> 
> Am 05/07/2022 um 10:17 schrieb Emanuele Giuseppe Esposito:
>>
>>
>> Am 05/07/2022 um 10:14 schrieb Stefan Hajnoczi:
>>> On Wed, Jun 29, 2022 at 10:15:31AM -0400, Emanuele Giuseppe Esposito wrote:
>>>> diff --git a/blockdev.c b/blockdev.c
>>>> index 71f793c4ab..5b79093155 100644
>>>> --- a/blockdev.c
>>>> +++ b/blockdev.c
>>>> @@ -150,12 +150,15 @@ void blockdev_mark_auto_del(BlockBackend *blk)
>>>>           return;
>>>>       }
>>>>   
>>>> -    for (job = block_job_next(NULL); job; job = block_job_next(job)) {
>>>> +    JOB_LOCK_GUARD();
>>>> +
>>>> +    for (job = block_job_next_locked(NULL); job;
>>>> +         job = block_job_next_locked(job)) {
>>>>           if (block_job_has_bdrv(job, blk_bs(blk))) {
>>>>               AioContext *aio_context = job->job.aio_context;
>>>>               aio_context_acquire(aio_context);
>>>
>>> Is there a lock ordering rule for job_mutex and the AioContext lock? I
>>> haven't audited the code, but there might be ABBA lock ordering issues.
>>
>> Doesn't really matter here, as lock is nop. To be honest I forgot which
>> one should go first, probably job_lock because the aiocontext lock can
>> be taken and released in callbacks.
>>
>> Should I resend with ordering fixed? Just to have a consistent logic
> 
> Well actually how do I fix that? I would just add useless additional
> changes into the diff, because for example in the case below I am not
> even sure what exactly is the aiocontext protecting.
> 
> So I guess I'll leave as it is. I will just update the commit message to
> make sure it is clear that the lock is nop and ordering is mixed.
> 

Yes, I think it's OK.

As far as I understand, our final ordering rule is that job_mutex can be taken under aio context lock but not visa-versa.

Still, there some aio-context-lock critical sections that are inside job_mutex-lock critical section during the series, just because we don't know the way to avoid it except just merge almost the whole series into one patch. That's why job_mutex is a noop during the series and should become real mutex in the same time with removing these aio-context-lock critical section which breaks the ordering rule.


-- 
Best regards,
Vladimir