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Date:   Mon, 22 Jul 2019 12:13:40 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        Matthew Wilcox <willy@infradead.org>, aarcange@redhat.com,
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Subject: Re: RFC: call_rcu_outstanding (was Re: WARNING in __mmdrop)
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On Mon, Jul 22, 2019 at 08:55:34AM -0700, Paul E. McKenney wrote:
> On Mon, Jul 22, 2019 at 11:47:24AM -0400, Michael S. Tsirkin wrote:
> > On Mon, Jul 22, 2019 at 11:14:39AM -0400, Joel Fernandes wrote:
> > > [snip]
> > > > > Would it make sense to have call_rcu() check to see if there are many
> > > > > outstanding requests on this CPU and if so process them before returning?
> > > > > That would ensure that frequent callers usually ended up doing their
> > > > > own processing.
> > > 
> > > Other than what Paul already mentioned about deadlocks, I am not sure if this
> > > would even work for all cases since call_rcu() has to wait for a grace
> > > period.
> > > 
> > > So, if the number of outstanding requests are higher than a certain amount,
> > > then you *still* have to wait for some RCU configurations for the grace
> > > period duration and cannot just execute the callback in-line. Did I miss
> > > something?
> > > 
> > > Can waiting in-line for a grace period duration be tolerated in the vhost case?
> > > 
> > > thanks,
> > > 
> > >  - Joel
> > 
> > No, but it has many other ways to recover (try again later, drop a
> > packet, use a slower copy to/from user).
> 
> True enough!  And your idea of taking recovery action based on the number
> of callbacks seems like a good one while we are getting RCU's callback
> scheduling improved.
> 
> By the way, was this a real problem that you could make happen on real
> hardware?


>  If not, I would suggest just letting RCU get improved over
> the next couple of releases.


So basically use kfree_rcu but add a comment saying e.g. "WARNING:
in the future callers of kfree_rcu might need to check that
not too many callbacks get queued. In that case, we can
disable the optimization, or recover in some other way.
Watch this space."


> If it is something that you actually made happen, please let me know
> what (if anything) you need from me for your callback-counting EBUSY
> scheme.
> 
> 							Thanx, Paul

If you mean kfree_rcu causing OOM then no, it's all theoretical.
If you mean synchronize_rcu stalling to the point where guest will OOPs,
then yes, that's not too hard to trigger.