Messages in this thread | | | From | Muchun Song <> | Date | Thu, 19 Aug 2021 10:04:06 +0800 | Subject | Re: [PATCH v3] mm/mempolicy: fix a race between offset_il_node and mpol_rebind_task |
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On Wed, Aug 18, 2021 at 11:07 PM Matthew Wilcox <willy@infradead.org> wrote: > > 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@infradead.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.
Here barrier() is just a compiler barrier, which is cheaper than smp_rmb() which usually equals to memory barrier instruction plus barrier(). So I think barrier() , which will stabilize the nodemask in a register or on the stack, is more appropriate here.
Thanks.
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