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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <50460F3F.8080305@redhat.com>
Date:	Tue, 04 Sep 2012 16:25:03 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Michael S. Tsirkin" <mst@...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

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/