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

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Message-Id: <20190722161555.GJ14271@linux.ibm.com>
Date:   Mon, 22 Jul 2019 09:15:55 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
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 01:04:48PM -0300, Jason Gunthorpe wrote:
> On Mon, Jul 22, 2019 at 08:52:35AM -0700, Paul E. McKenney wrote:
> > So why then is there a problem?
> 
> I'm not sure there is a real problem, I thought Michael was just
> asking how to design with RCU in the case where the user controls the
> kfree_rcu??
> 
> Sounds like the answer is "don't worry about it" ?

Unless you can force failures, you should be good.

And either way, improvements to RCU's handling of this sort of situation
are in the works.  And rcutorture has gained tests of this stuff in the
last year or so as well, see its "fwd_progress" module parameter and
the related code.

							Thanx, Paul