linux-kernel - Re: [PATCH -next] mm/percpu: fix data-race with pcpu_nr_empty_pop

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Message-ID: <YXdq7+PJqGLCu4nH@fedora>
Date:   Mon, 25 Oct 2021 22:41:51 -0400
From:   Dennis Zhou <dennis@...nel.org>
To:     Christoph Lameter <cl@...two.de>
Cc:     Yuanzheng Song <songyuanzheng@...wei.com>, dennis@...nel.org,
        tj@...nel.org, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] mm/percpu: fix data-race with
 pcpu_nr_empty_pop_pages

Hello,

On Mon, Oct 25, 2021 at 09:50:48AM +0200, Christoph Lameter wrote:
> On Mon, 25 Oct 2021, Yuanzheng Song wrote:
> 
> > When reading the pcpu_nr_empty_pop_pages in pcpu_alloc()
> > and writing the pcpu_nr_empty_pop_pages in
> > pcpu_update_empty_pages() at the same time,
> > the data-race occurs.
> 
> Looks like a use case for the atomic RMV instructions.
> 

Yeah. I see 2 options. Switch the variable over to an atomic or we can
move the read behind pcpu_lock. All the writes are already behind it
othewise that would actually be problematic. In this particular case,
reading a wrong # of empty pages isn't a big deal as eventually the
background work will get scheduled.

Thanks,
Dennis

> > To fix this issue, use READ_ONCE() and WRITE_ONCE() to
> > read and write the pcpu_nr_empty_pop_pages.
> 
> Never thought that READ_ONCE and WRITE_ONCE can fix races like
> this. Really?
> 
> > diff --git a/mm/percpu.c b/mm/percpu.c
> > index 293009cc03ef..e8ef92e698ab 100644
> > --- a/mm/percpu.c
> > +++ b/mm/percpu.c
> > @@ -574,7 +574,9 @@ static void pcpu_isolate_chunk(struct pcpu_chunk *chunk)
> >
> >  	if (!chunk->isolated) {
> >  		chunk->isolated = true;
> > -		pcpu_nr_empty_pop_pages -= chunk->nr_empty_pop_pages;
> > +		WRITE_ONCE(pcpu_nr_empty_pop_pages,
> > +			   READ_ONCE(pcpu_nr_empty_pop_pages) -
> > +			   chunk->nr_empty_pop_pages);
> 
> atomic_sub()?
> 
> >  	}
> >  	list_move(&chunk->list, &pcpu_chunk_lists[pcpu_to_depopulate_slot]);
> >  }
> > @@ -585,7 +587,9 @@ static void pcpu_reintegrate_chunk(struct pcpu_chunk *chunk)
> >
> >  	if (chunk->isolated) {
> >  		chunk->isolated = false;
> > -		pcpu_nr_empty_pop_pages += chunk->nr_empty_pop_pages;
> > +		WRITE_ONCE(pcpu_nr_empty_pop_pages,
> > +			   READ_ONCE(pcpu_nr_empty_pop_pages) +
> > +			   chunk->nr_empty_pop_pages);
> 
> atomic_add()?
>