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

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Date:   Mon, 10 Feb 2020 21:44:58 +0100
From:   Marco Elver <elver@...gle.com>
To:     Qian Cai <cai@....pw>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] mm/filemap: fix a data race in filemap_fault()

On Mon, 10 Feb 2020 at 21:28, Qian Cai <cai@....pw> wrote:
>
> 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.

Note: the fact that we assume writes aligned up to word-size are
atomic is based on current preferences we were told about. Just
because the tool won't complain right now (although a simple config
switch will make it complain again), we don't want to forget the
writes entirely. If it is a simple write, do the WRITE_ONCE if it
makes sense. I, for one, still can't prove if all compilers won't
screw up a write due to an omitted WRITE_ONCE somehow. [Yes, for more
complex ops like 'var++', turning them into 'WRITE_ONCE(var, var + 1)'
isn't as readable, so these are a bit tricky until we get primitives
to properly deal with them.]