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

[Michael S. Tsirkin]

On Tue, Sep 04, 2012 at 01:18:31PM +0200, Paolo Bonzini wrote:
> Il 04/09/2012 13:09, Michael S. Tsirkin ha scritto:
> >> >     queuecommand on CPU #0         queuecommand #2 on CPU #1
> >> >   --------------------------------------------------------------
> >> >     atomic_inc_return(...) == 1
> >> >                                    atomic_inc_return(...) == 2
> >> >                                    virtscsi_queuecommand to queue #1
> >> >     tgt->req_vq = queue #0
> >> >     virtscsi_queuecommand to queue #0
> >> > 
> >> > then two requests are issued to different queues without a quiescent
> >> > point in the middle.
> > What happens then? Does this break correctness?
> 
> Yes, requests to the same target should be processed in FIFO order, or
> you have things like a flush issued before the write it was supposed to
> flush.  This is why I can only change the queue when there is no request
> pending.
> 
> Paolo

I see.  I guess you can rewrite this as:
atomic_inc
if (atomic_read() == 1)
which is a bit cheaper, and make the fact
that you do not need increment and return to be atomic,
explicit.

Another simple idea: store last processor id in target,
if it is unchanged no need to play with req_vq
and take spinlock.

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.




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