Re: [RFC PATCH v3 2/8] vhost: annotate virtqueue access lock

[David Marchand <david.marchand@redhat.com>]

On Thu, Apr 21, 2022 at 5:25 PM Maxime Coquelin
<maxime.coquelin@redhat.com> wrote:
> On 4/11/22 13:00, David Marchand wrote:
> > This change simply annotates existing paths of the code leading to
> > manipulations of the vq->access_lock.
> >
> > One small change is required: vhost_poll_enqueue_completed was getting
> > a queue_id to get hold of the vq, while its callers already knew of
> > the vq. For the annotation sake, vq is now directly passed.
>
> It is anyway more consistent with the rest of the code to pass directly
> the vq in internal API when queue ID is not needed.
>
> > vhost_user_lock_all_queue_pairs and vhost_user_unlock_all_queue_pairs
> > are skipped since vq->access_lock are conditionally held.
>
> As discussed off-list, I wonder whether it could be possible to rework
> the conditional lock holding using the static array and some macros so
> that we could statically specify for each request if the lock is
> required.

We did discuss some ideas off-list, but in the end, since we have
multiple locks being dynamically taken in
vhost_user_lock_all_queue_pairs, I see no way to statically annotate
the code.

We could rework the code to have message handlers in a consolidated
static array, but that would not help with annotations.
I had some patches going in that direction (related to some fd fixes I
sent before), but it needs more work.
I'll see if I can send this later in the release or it will go to next release.


-- 
David Marchand