Re: WARNING in __mmdrop

[Jason Wang]

On 2019/7/25 äå1:52, Michael S. Tsirkin wrote:
> On Tue, Jul 23, 2019 at 09:31:35PM +0800, Jason Wang wrote:
> > On 2019/7/23 äå5:26, Michael S. Tsirkin wrote:
> > > On Tue, Jul 23, 2019 at 04:49:01PM +0800, Jason Wang wrote:
> > > > On 2019/7/23 äå4:10, Michael S. Tsirkin wrote:
> > > > > On Tue, Jul 23, 2019 at 03:53:06PM +0800, Jason Wang wrote:
> > > > > > On 2019/7/23 äå3:23, 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?


Yes, but it's not the only lock. I don't remember the details, but I can 
confirm I meet issues with one or two other locks.


>    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);


The problem is the vq address could be changed at this time.


> 		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.

Consider we switch to use kfree_rcu(), what's the advantage of per-vq SRCU?

Thanks