From: Paolo Bonzini <pbonzini@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: "Nicholas A. Bellinger" <nab@linux-iscsi.org>,
linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org,
kvm@vger.kernel.org, rusty@rustcorp.com.au, jasowang@redhat.com,
virtualization@lists.linux-foundation.org,
Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
target-devel <target-devel@vger.kernel.org>
Subject: Re: [PATCH 5/5] virtio-scsi: introduce multiqueue support
Date: Tue, 04 Sep 2012 16:25:03 +0200 [thread overview]
Message-ID: <50460F3F.8080305@redhat.com> (raw)
In-Reply-To: <20120904141924.GK9805@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
next prev parent reply other threads:[~2012-09-04 14:25 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-08-28 11:54 [PATCH 0/5] Multiqueue virtio-scsi Paolo Bonzini
2012-08-28 11:54 ` [PATCH 1/5] virtio-ring: move queue_index to vring_virtqueue Paolo Bonzini
2012-08-29 7:54 ` Jason Wang
2012-09-05 23:32 ` Rusty Russell
2012-08-28 11:54 ` [PATCH 2/5] virtio: introduce an API to set affinity for a virtqueue Paolo Bonzini
2012-09-05 23:32 ` Rusty Russell
2012-08-28 11:54 ` [PATCH 3/5] virtio-scsi: allocate target pointers in a separate memory block Paolo Bonzini
2012-08-28 14:07 ` Sasha Levin
2012-08-28 14:25 ` Paolo Bonzini
2012-08-28 11:54 ` [PATCH 4/5] virtio-scsi: pass struct virtio_scsi to virtqueue completion function Paolo Bonzini
2012-08-28 11:54 ` [PATCH 5/5] virtio-scsi: introduce multiqueue support Paolo Bonzini
2012-09-04 2:21 ` Nicholas A. Bellinger
2012-09-04 6:46 ` Paolo Bonzini
2012-09-04 8:46 ` Michael S. Tsirkin
2012-09-04 10:25 ` Paolo Bonzini
2012-09-04 11:09 ` Michael S. Tsirkin
2012-09-04 11:18 ` Paolo Bonzini
2012-09-04 13:35 ` Michael S. Tsirkin
2012-09-04 13:45 ` Paolo Bonzini
2012-09-04 14:19 ` Michael S. Tsirkin
2012-09-04 14:25 ` Paolo Bonzini [this message]
2012-09-04 20:11 ` Nicholas A. Bellinger
2012-09-05 7:03 ` Paolo Bonzini
2012-09-04 12:48 ` Michael S. Tsirkin
2012-09-04 13:49 ` Paolo Bonzini
2012-09-04 14:21 ` Michael S. Tsirkin
2012-09-04 14:30 ` Paolo Bonzini
2012-09-04 14:41 ` Michael S. Tsirkin
2012-09-04 14:47 ` Michael S. Tsirkin
2012-09-04 14:55 ` Paolo Bonzini
2012-09-04 15:03 ` Michael S. Tsirkin
2012-08-30 7:13 ` [PATCH 0/5] Multiqueue virtio-scsi Stefan Hajnoczi
2012-08-30 14:53 ` Michael S. Tsirkin
2012-08-30 15:45 ` Paolo Bonzini
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