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

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Date:   Mon, 10 Feb 2020 15:28:49 -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 11:21 -0800, Matthew Wilcox wrote:
> On Mon, Feb 10, 2020 at 02:20:48PM -0500, Qian Cai wrote:
> > 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).
> 
> My point is that this is a legitimate warning from the sanitiser.
> The point of your patches should not be to remove all the warnings!

The KCSAN will assume the write is "atomic" if it is aligned and within word-
size which is the case for "ra->mmap_miss", so I somehow skip auditing the
locking around the concurrent writers, but I got your point. Next time, I'll
spend a bit more time looking.