Re: [PATCH 5/5] virtio-scsi: introduce multiqueue support

[Paolo Bonzini <pbonzini@redhat.com>]

Il 04/09/2012 16:19, Michael S. Tsirkin ha scritto:
> > > Also - some kind of comment explaining why a similar race can not happen
> > > with this lock in place would be nice: I see why this specific race can
> > > not trigger but since lock is dropped later before you submit command, I
> > > have hard time convincing myself what exactly gurantees that vq is never
> > > switched before or even while command is submitted.
> > 
> > Because tgt->reqs will never become zero (which is a necessary condition
> > for tgt->req_vq to change), as long as one request is executing
> > virtscsi_queuecommand.
> 
> Yes but this logic would apparently imply the lock is not necessary, and
> it actually is. I am not saying anything is wrong just that it
> looks scary.

Ok, I get the misunderstanding.  For the logic to hold, you need a
serialization point after which tgt->req_vq is not changed.  The lock
provides one such serialization point: after you unlock tgt->tgt_lock,
nothing else will change tgt->req_vq until your request completes.

Without the lock, there could always be a thread that is in the "then"
branch but has been scheduled out, and when rescheduled it will change
tgt->req_vq.

Perhaps the confusion comes from the atomic_inc_return, and that was
what my "why is this atomic" wanted to clear.  **tgt->reqs is only
atomic to avoid taking a spinlock in the ISR**.  If you read the code
with the lock, but with tgt->reqs as a regular non-atomic int, it should
be much easier to reason on the code.  I can split the patch if needed.

Paolo