linux-kernel - Re: RFC: call_rcu_outstanding (was Re: WARNING in _

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Date:   Mon, 22 Jul 2019 11:47:24 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Matthew Wilcox <willy@...radead.org>, aarcange@...hat.com,
        akpm@...ux-foundation.org, christian@...uner.io,
        davem@...emloft.net, ebiederm@...ssion.com,
        elena.reshetova@...el.com, guro@...com, hch@...radead.org,
        james.bottomley@...senpartnership.com, jasowang@...hat.com,
        jglisse@...hat.com, keescook@...omium.org, ldv@...linux.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-parisc@...r.kernel.org,
        luto@...capital.net, mhocko@...e.com, mingo@...nel.org,
        namit@...are.com, peterz@...radead.org,
        syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk,
        wad@...omium.org
Subject: Re: RFC: call_rcu_outstanding (was Re: WARNING in __mmdrop)

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).

-- 
MST