From: "Michael S. Tsirkin" <mst@redhat.com>
To: Paolo Bonzini <pbonzini@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, 4 Sep 2012 17:19:24 +0300 [thread overview]
Message-ID: <20120904141924.GK9805@redhat.com> (raw)
In-Reply-To: <50460615.3000006@redhat.com>
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
WARNING: multiple messages have this Message-ID (diff)
From: "Michael S. Tsirkin" <mst@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>,
linux-scsi@vger.kernel.org, kvm@vger.kernel.org,
linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org,
target-devel <target-devel@vger.kernel.org>,
Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 5/5] virtio-scsi: introduce multiqueue support
Date: Tue, 4 Sep 2012 17:19:24 +0300 [thread overview]
Message-ID: <20120904141924.GK9805@redhat.com> (raw)
In-Reply-To: <50460615.3000006@redhat.com>
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
next prev parent reply other threads:[~2012-09-04 14:18 UTC|newest]
Thread overview: 68+ 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 ` Paolo Bonzini
2012-08-28 11:54 ` [PATCH 1/5] virtio-ring: move queue_index to vring_virtqueue Paolo Bonzini
2012-08-28 11:54 ` Paolo Bonzini
2012-08-29 7:54 ` Jason Wang
2012-08-29 7:54 ` Jason Wang
2012-09-05 23:32 ` Rusty Russell
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
2012-08-28 11:54 ` Paolo Bonzini
2012-09-05 23:32 ` Rusty Russell
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 11:54 ` Paolo Bonzini
2012-08-28 14:07 ` Sasha Levin
2012-08-28 14:07 ` Sasha Levin
2012-08-28 14:25 ` Paolo Bonzini
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 ` Paolo Bonzini
2012-08-28 11:54 ` [PATCH 5/5] virtio-scsi: introduce multiqueue support Paolo Bonzini
2012-08-28 11:54 ` Paolo Bonzini
2012-09-04 2:21 ` Nicholas A. Bellinger
2012-09-04 2:21 ` Nicholas A. Bellinger
2012-09-04 6:46 ` Paolo Bonzini
2012-09-04 6:46 ` Paolo Bonzini
2012-09-04 8:46 ` Michael S. Tsirkin
2012-09-04 8:46 ` Michael S. Tsirkin
2012-09-04 10:25 ` Paolo Bonzini
2012-09-04 10:25 ` Paolo Bonzini
2012-09-04 11:09 ` Michael S. Tsirkin
2012-09-04 11:09 ` Michael S. Tsirkin
2012-09-04 11:18 ` Paolo Bonzini
2012-09-04 11:18 ` Paolo Bonzini
2012-09-04 13:35 ` Michael S. Tsirkin
2012-09-04 13:35 ` Michael S. Tsirkin
2012-09-04 13:45 ` Paolo Bonzini
2012-09-04 13:45 ` Paolo Bonzini
2012-09-04 14:19 ` Michael S. Tsirkin [this message]
2012-09-04 14:19 ` Michael S. Tsirkin
2012-09-04 14:25 ` Paolo Bonzini
2012-09-04 14:25 ` Paolo Bonzini
2012-09-04 20:11 ` Nicholas A. Bellinger
2012-09-04 20:11 ` Nicholas A. Bellinger
2012-09-05 7:03 ` Paolo Bonzini
2012-09-05 7:03 ` Paolo Bonzini
2012-09-04 12:48 ` Michael S. Tsirkin
2012-09-04 12:48 ` Michael S. Tsirkin
2012-09-04 13:49 ` Paolo Bonzini
2012-09-04 13:49 ` Paolo Bonzini
2012-09-04 14:21 ` Michael S. Tsirkin
2012-09-04 14:21 ` Michael S. Tsirkin
2012-09-04 14:30 ` Paolo Bonzini
2012-09-04 14:30 ` Paolo Bonzini
2012-09-04 14:41 ` Michael S. Tsirkin
2012-09-04 14:41 ` Michael S. Tsirkin
2012-09-04 14:47 ` Michael S. Tsirkin
2012-09-04 14:47 ` Michael S. Tsirkin
2012-09-04 14:55 ` Paolo Bonzini
2012-09-04 14:55 ` Paolo Bonzini
2012-09-04 15:03 ` Michael S. Tsirkin
2012-09-04 15:03 ` Michael S. Tsirkin
2012-08-30 7:13 ` [PATCH 0/5] Multiqueue virtio-scsi Stefan Hajnoczi
2012-08-30 7:13 ` Stefan Hajnoczi
2012-08-30 14:53 ` Michael S. Tsirkin
2012-08-30 14:53 ` Michael S. Tsirkin
2012-08-30 15:45 ` Paolo Bonzini
2012-08-30 15:45 ` Paolo Bonzini
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20120904141924.GK9805@redhat.com \
--to=mst@redhat.com \
--cc=axboe@kernel.dk \
--cc=hch@lst.de \
--cc=jasowang@redhat.com \
--cc=kvm@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-scsi@vger.kernel.org \
--cc=nab@linux-iscsi.org \
--cc=pbonzini@redhat.com \
--cc=rusty@rustcorp.com.au \
--cc=target-devel@vger.kernel.org \
--cc=virtualization@lists.linux-foundation.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.