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

[Vladimir Sementsov-Ogievskiy]

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