Re: [PATCH] virtiofsd: vu_dispatch locking should never fail

[Greg Kurz]

On Wed, 3 Feb 2021 14:57:23 +0000
Stefan Hajnoczi <stefanha@redhat.com> wrote:

> On Fri, Jan 29, 2021 at 04:53:12PM +0100, Greg Kurz wrote:
> > pthread_rwlock_rdlock() and pthread_rwlock_wrlock() can fail if a
> > deadlock condition is detected or the current thread already owns
> > the lock. They can also fail, like pthread_rwlock_unlock(), if the
> > mutex wasn't properly initialized. None of these are ever expected
> > to happen with fv_VuDev::vu_dispatch_rwlock.
> > 
> > Some users already check the return value and assert, some others
> > don't. Introduce rdlock/wrlock/unlock wrappers that just do the
> > former and use them everywhere.
> > 
> > Signed-off-by: Greg Kurz <groug@kaod.org>
> 
> What is the purpose of this commit:
> 1. Code cleanup/consistency?
> 2. Helps debug an issue that doesn't occur in production but you hit
>    during development?
> 3. Needed to diagnose a real-world issue? How do you reproduce it?
> 
> I wanted to check to make sure I'm not missing an issue you found with
> production workloads.
> 

I would have provided more details if that came from an actual issue,
but you're definitely right to ask : this falls more under 1.

> > @@ -645,8 +662,7 @@ static void *fv_queue_thread(void *opaque)
> >              break;
> >          }
> >          /* Mutual exclusion with virtio_loop() */
> > -        ret = pthread_rwlock_rdlock(&qi->virtio_dev->vu_dispatch_rwlock);
> > -        assert(ret == 0); /* there is no possible error case */
> > +        vu_dispatch_wrlock(qi->virtio_dev);
> 
> s/vu_dispatch_wrlock/vu_dispatch_rdlock/ ?

Oops... definitely...

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