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Date:   Thu, 25 Jul 2019 01:52:53 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
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Subject: Re: WARNING in __mmdrop
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On Tue, Jul 23, 2019 at 09:31:35PM +0800, Jason Wang wrote:
> 
> On 2019/7/23 下午5:26, Michael S. Tsirkin wrote:
> > On Tue, Jul 23, 2019 at 04:49:01PM +0800, Jason Wang wrote:
> > > On 2019/7/23 下午4:10, Michael S. Tsirkin wrote:
> > > > On Tue, Jul 23, 2019 at 03:53:06PM +0800, Jason Wang wrote:
> > > > > On 2019/7/23 下午3:23, Michael S. Tsirkin wrote:
> > > > > > > > Really let's just use kfree_rcu. It's way cleaner: fire and forget.
> > > > > > > Looks not, you need rate limit the fire as you've figured out?
> > > > > > See the discussion that followed. Basically no, it's good enough
> > > > > > already and is only going to be better.
> > > > > > 
> > > > > > > And in fact,
> > > > > > > the synchronization is not even needed, does it help if I leave a comment to
> > > > > > > explain?
> > > > > > Let's try to figure it out in the mail first. I'm pretty sure the
> > > > > > current logic is wrong.
> > > > > Here is what the code what to achieve:
> > > > > 
> > > > > - The map was protected by RCU
> > > > > 
> > > > > - Writers are: MMU notifier invalidation callbacks, file operations (ioctls
> > > > > etc), meta_prefetch (datapath)
> > > > > 
> > > > > - Readers are: memory accessor
> > > > > 
> > > > > Writer are synchronized through mmu_lock. RCU is used to synchronized
> > > > > between writers and readers.
> > > > > 
> > > > > The synchronize_rcu() in vhost_reset_vq_maps() was used to synchronized it
> > > > > with readers (memory accessors) in the path of file operations. But in this
> > > > > case, vq->mutex was already held, this means it has been serialized with
> > > > > memory accessor. That's why I think it could be removed safely.
> > > > > 
> > > > > Anything I miss here?
> > > > > 
> > > > So invalidate callbacks need to reset the map, and they do
> > > > not have vq mutex. How can they do this and free
> > > > the map safely? They need synchronize_rcu or kfree_rcu right?
> > > Invalidation callbacks need but file operations (e.g ioctl) not.
> > > 
> > > 
> > > > And I worry somewhat that synchronize_rcu in an MMU notifier
> > > > is a problem, MMU notifiers are supposed to be quick:
> > > Looks not, since it can allow to be blocked and lots of driver depends on
> > > this. (E.g mmu_notifier_range_blockable()).
> > Right, they can block. So why don't we take a VQ mutex and be
> > done with it then? No RCU tricks.
> 
> 
> This is how I want to go with RFC and V1. But I end up with deadlock between
> vq locks and some MM internal locks. So I decide to use RCU which is 100%
> under the control of vhost.
> 
> Thanks

And I guess the deadlock is because GUP is taking mmu locks which are
taken on mmu notifier path, right?  How about we add a seqlock and take
that in invalidate callbacks?  We can then drop the VQ lock before GUP,
and take it again immediately after.

something like
	if (!vq_meta_mapped(vq)) {
		vq_meta_setup(&uaddrs);
		mutex_unlock(vq->mutex)
		vq_meta_map(&uaddrs);
		mutex_lock(vq->mutex)

		/* recheck both sock->private_data and seqlock count. */
		if changed - bail out
	}

And also requires that VQ uaddrs is defined like this:
- writers must have both vq mutex and dev mutex
- readers must have either vq mutex or dev mutex


That's a big change though. For now, how about switching to a per-vq SRCU?
That is only a little bit more expensive than RCU, and we
can use synchronize_srcu_expedited.

-- 
MST