On Fri, Jun 25, 2010 at 12:39:21PM +0930, Rusty Russell wrote: > On Thu, 24 Jun 2010 03:00:30 pm Stefan Hajnoczi wrote: > > On Wed, Jun 23, 2010 at 11:12 PM, Anthony Liguori <anthony@xxxxxxxxxxxxx> wrote: > > > Shouldn't it be possible to just drop the lock before invoking > > > virtqueue_kick() and reacquire it afterwards? There's nothing in that > > > virtqueue_kick() path that the lock is protecting AFAICT. > > > > No, that would lead to a race condition because vq->num_added is > > modified by both virtqueue_add_buf_gfp() and virtqueue_kick(). > > Without a lock held during virtqueue_kick() another vcpu could add > > bufs while vq->num_added is used and cleared by virtqueue_kick(): > > Right, this dovetails with another proposed change (was it Michael?) > where we would update the avail idx inside add_buf, rather than waiting > until kick. This means a barrier inside add_buf, but that's probably > fine. > > If we do that, then we don't need a lock on virtqueue_kick. > > Michael, thoughts? Maybe not even that: I think we could just do virtio_wmb() in add, and keep the mb() in kick. What I'm a bit worried about is contention on the cacheline including index and flags: the more we write to that line, the worse it gets. So need to test performance impact of this change: I didn't find time to do this yet, as I am trying to finalize the used index publishing patches. Any takers? Do we see performance improvement after making kick lockless? > Thanks, > Rusty. > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
