Re: [PATCH] mm: fix a data race in put_page()

[Marco Elver]

On Mon, 10 Feb 2020 at 14:36, Qian Cai <cai@xxxxxx> wrote:
>
> On Mon, 2020-02-10 at 13:58 +0100, Marco Elver wrote:
> > On Mon, 10 Feb 2020 at 13:16, Qian Cai <cai@xxxxxx> wrote:
> > >
> > >
> > >
> > > > On Feb 10, 2020, at 2:48 AM, Marco Elver <elver@xxxxxxxxxx> wrote:
> > > >
> > > > Here is an alternative:
> > > >
> > > > Let's say KCSAN gives you this:
> > > >   /* ... Assert that the bits set in mask are not written
> > > > concurrently; they may still be read concurrently.
> > > >     The access that immediately follows is assumed to access those
> > > > bits and safe w.r.t. data races.
> > > >
> > > >     For example, this may be used when certain bits of @flags may
> > > > only be modified when holding the appropriate lock,
> > > >     but other bits may still be modified locklessly.
> > > >   ...
> > > >  */
> > > >   #define ASSERT_EXCLUSIVE_BITS(flags, mask)   ....
> > > >
> > > > Then we can write page_zonenum as follows:
> > > >
> > > > static inline enum zone_type page_zonenum(const struct page *page)
> > > > {
> > > > +       ASSERT_EXCLUSIVE_BITS(page->flags, ZONES_MASK << ZONES_PGSHIFT);
> > > >        return (page->flags >> ZONES_PGSHIFT) & ZONES_MASK;
> > > > }
> > > >
> > > > This will accomplish the following:
> > > > 1. The current code is not touched, and we do not have to verify that
> > > > the change is correct without KCSAN.
> > > > 2. We're not introducing a bunch of special macros to read bits in various ways.
> > > > 3. KCSAN will assume that the access is safe, and no data race report
> > > > is generated.
> > > > 4. If somebody modifies ZONES bits concurrently, KCSAN will tell you
> > > > about the race.
> > > > 5. We're documenting the code.
> > > >
> > > > Anything I missed?
> > >
> > > I donât know. Having to write the same line twice does not feel me any better than data_race() with commenting occasionally.
> >
> > Point 4 above: While data_race() will ignore cause KCSAN to not report
> > the data race, now you might be missing a real bug: if somebody
> > concurrently modifies the bits accessed, you want to know about it!
> > Either way, it's up to you to add the ASSERT_EXCLUSIVE_BITS, but just
> > remember that if you decide to silence it with data_race(), you need
> > to be sure there are no concurrent writers to those bits.
>
> Right, in this case, there is no concurrent writers to those bits, so I'll add a
> comment should be sufficient. However, I'll keep ASSERT_EXCLUSIVE_BITS() in mind
> for other places.

Right now there are no concurrent writers to those bits. But somebody
might introduce a bug that will write them, even though they shouldn't
have. With ASSERT_EXCLUSIVE_BITS() you can catch that. Once I have the
patches for this out, I would consider adding it here for this reason.

> >
> > There is no way to automatically infer all over the kernel which bits
> > we care about, and the most reliable is to be explicit about it. I
> > don't see a problem with it per se.
> >
> > Thanks,
> > -- Marco