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

[Stefan Hajnoczi]

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.

> @@ -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/ ?

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