linux-kernel - Re: [PATCH v3] mm/mempolicy: fix a race between offset_il_node and mpol_rebind

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Date:   Wed, 18 Aug 2021 16:07:11 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        yanghui <yanghui.def@...edance.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] mm/mempolicy: fix a race between offset_il_node and
 mpol_rebind_task

On Wed, Aug 18, 2021 at 10:02:46PM +0800, Muchun Song wrote:
> On Tue, Aug 17, 2021 at 9:43 AM Matthew Wilcox <willy@...radead.org> wrote:
> > > > +   unsigned int target, nnodes;
> > > >     int i;
> > > >     int nid;
> > > > +   /*
> > > > +    * The barrier will stabilize the nodemask in a register or on
> > > > +    * the stack so that it will stop changing under the code.
> > > > +    *
> > > > +    * Between first_node() and next_node(), pol->nodes could be changed
> > > > +    * by other threads. So we put pol->nodes in a local stack.
> > > > +    */
> > > > +   barrier();
> >
> > I think this could be an smp_rmb()?
> 
> Hi Matthew,
> 
> I have a question. Why is barrier() not enough?

I think barrier() may be more than is necessary.  We don't need a
barrier on non-SMP systems (or do we?)  And we only need to order reads,
not writes.