linux-kernel - Re: [PATCH -next] mm/filemap: fix a data race in filemap

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Date:   Mon, 10 Feb 2020 14:20:48 -0500
From:   Qian Cai <cai@....pw>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     akpm@...ux-foundation.org, elver@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] mm/filemap: fix a data race in filemap_fault()

On Mon, 2020-02-10 at 09:25 -0800, Matthew Wilcox wrote:
> On Mon, Feb 10, 2020 at 12:00:29PM -0500, Qian Cai wrote:
> > @@ -2622,7 +2622,7 @@ void filemap_map_pages(struct vm_fault *vmf,
> >  		if (page->index >= max_idx)
> >  			goto unlock;
> >  
> > -		if (file->f_ra.mmap_miss > 0)
> > +		if (data_race(file->f_ra.mmap_miss > 0))
> >  			file->f_ra.mmap_miss--;
> 
> How is this safe?  Two threads can each see 1, and then both decrement the
> in-memory copy, causing it to end up at -1.

Well, I meant to say it is safe from *data* races rather than all other races,
but it is a good catch for the underflow cases and makes some sense to fix them
together (so we don't need to touch the same lines over and over again).