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

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Message-ID: <20120904141924.GK9805@redhat.com>
Date:	Tue, 4 Sep 2012 17:19:24 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	kvm@...r.kernel.org, rusty@...tcorp.com.au, jasowang@...hat.com,
	virtualization@...ts.linux-foundation.org,
	Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
	target-devel <target-devel@...r.kernel.org>
Subject: Re: [PATCH 5/5] virtio-scsi: introduce multiqueue support

On Tue, Sep 04, 2012 at 03:45:57PM +0200, Paolo Bonzini wrote:
> > 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.
> 
> Paolo

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.

-- 
MST
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