Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker

[Jason Gunthorpe]

On Thu, Aug 01, 2019 at 01:02:18PM +0800, Jason Wang wrote:
> 
> On 2019/8/1 äå3:30, Jason Gunthorpe wrote:
> > On Wed, Jul 31, 2019 at 09:28:20PM +0800, Jason Wang wrote:
> > > On 2019/7/31 äå8:39, Jason Gunthorpe wrote:
> > > > On Wed, Jul 31, 2019 at 04:46:53AM -0400, Jason Wang wrote:
> > > > > We used to use RCU to synchronize MMU notifier with worker. This leads
> > > > > calling synchronize_rcu() in invalidate_range_start(). But on a busy
> > > > > system, there would be many factors that may slow down the
> > > > > synchronize_rcu() which makes it unsuitable to be called in MMU
> > > > > notifier.
> > > > > 
> > > > > A solution is SRCU but its overhead is obvious with the expensive full
> > > > > memory barrier. Another choice is to use seqlock, but it doesn't
> > > > > provide a synchronization method between readers and writers. The last
> > > > > choice is to use vq mutex, but it need to deal with the worst case
> > > > > that MMU notifier must be blocked and wait for the finish of swap in.
> > > > > 
> > > > > So this patch switches use a counter to track whether or not the map
> > > > > was used. The counter was increased when vq try to start or finish
> > > > > uses the map. This means, when it was even, we're sure there's no
> > > > > readers and MMU notifier is synchronized. When it was odd, it means
> > > > > there's a reader we need to wait it to be even again then we are
> > > > > synchronized.
> > > > You just described a seqlock.
> > > 
> > > Kind of, see my explanation below.
> > > 
> > > 
> > > > We've been talking about providing this as some core service from mmu
> > > > notifiers because nearly every use of this API needs it.
> > > 
> > > That would be very helpful.
> > > 
> > > 
> > > > IMHO this gets the whole thing backwards, the common pattern is to
> > > > protect the 'shadow pte' data with a seqlock (usually open coded),
> > > > such that the mmu notififer side has the write side of that lock and
> > > > the read side is consumed by the thread accessing or updating the SPTE.
> > > 
> > > Yes, I've considered something like that. But the problem is, mmu notifier
> > > (writer) need to wait for the vhost worker to finish the read before it can
> > > do things like setting dirty pages and unmapping page. It looks to me
> > > seqlock doesn't provide things like this.
> > The seqlock is usually used to prevent a 2nd thread from accessing the
> > VA while it is being changed by the mm. ie you use something seqlocky
> > instead of the ugly mmu_notifier_unregister/register cycle.
> 
> 
> Yes, so we have two mappings:
> 
> [1] vring address to VA
> [2] VA to PA
> 
> And have several readers and writers
> 
> 1) set_vring_num_addr(): writer of both [1] and [2]
> 2) MMU notifier: reader of [1] writer of [2]
> 3) GUP: reader of [1] writer of [2]
> 4) memory accessors: reader of [1] and [2]
> 
> Fortunately, 1) 3) and 4) have already synchronized through vq->mutex. We
> only need to deal with synchronization between 2) and each of the reset:
> Sync between 1) and 2): For mapping [1], I do
> mmu_notifier_unregister/register. This help to avoid holding any lock to do
> overlap check.

I suspect you could have done this with a RCU technique instead of
register/unregister.

> Sync between 2) and 4): For mapping [1], both are readers, no need any
> synchronization. For mapping [2], synchronize through RCU (or something
> simliar to seqlock).

You can't really use a seqlock, seqlocks are collision-retry locks,
and the semantic here is that invalidate_range_start *MUST* not
continue until thread doing #4 above is guarenteed no longer touching
the memory.

This must be a proper barrier, like a spinlock, mutex, or
synchronize_rcu.

And, again, you can't re-invent a spinlock with open coding and get
something better.

Jason