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

[Stefan Hajnoczi]

On Tue, Jul 05, 2022 at 04:22:41PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> 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.

I'm also fine with resolving the ordering in a later patch.

Stefan

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