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

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Message-ID: <20190722134152.GA13013@ziepe.ca>
Date:   Mon, 22 Jul 2019 10:41:52 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.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 04:51:49AM -0700, Paul E. McKenney wrote:

> > > > 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.
> > > 
> > > Unfortunately, no.  Here is a code fragment illustrating why:

That is only true in the general case though, kfree_rcu() doesn't have
this problem since we know what the callback is doing. In general a
caller of kfree_rcu() should not need to hold any locks while calling
it.

We could apply the same idea more generally and have some
'call_immediate_or_rcu()' which has restrictions on the caller's
context.

I think if we have some kind of problem here it would be better to
handle it inside the core code and only require that callers use the
correct RCU API.

I can think of many places where kfree_rcu() is being used under user
control..

Jason