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

[Qian Cai <cai@lca.pw>]

On Mon, 2020-02-10 at 08:48 +0100, Marco Elver wrote:
> On Sun, 9 Feb 2020 at 08:15, John Hubbard <jhubbard@nvidia.com> wrote:
> > 
> > On 2/8/20 7:10 PM, Qian Cai wrote:
> > > 
> > > 
> > > > On Feb 8, 2020, at 8:44 PM, John Hubbard <jhubbard@nvidia.com> wrote:
> > > > 
> > > > So it looks like we're probably stuck with having to annotate the code. Given
> > > > that, there is a balance between how many macros, and how much commenting. For
> > > > example, if there is a single macro (data_race, for example), then we'll need to
> > > > add comments for the various cases, explaining which data_race situation is
> > > > happening.
> > > 
> > > On the other hand, it is perfect fine of not commenting on each data_race() that most of times, people could run git blame to learn more details. Actually, no maintainers from various of subsystems asked for commenting so far.
> > > 
> > 
> > Well, maybe I'm looking at this wrong. I was thinking that one should attempt to
> > understand the code on the screen, and that's generally best--but here, maybe
> > "data_race" is just something that means "tool cruft", really. So mentally we
> > would move toward visually filtering out the data_race "key word".
> 
> One thing to note is that 'data_race()' points out concurrency, and
> that somebody has deemed that the code won't break even with data
> races. Somebody trying to understand or modify the code should ensure
> this will still be the case. So, 'data_race()' isn't just tool cruft.
> It's documentation for something that really isn't obvious from the
> code alone.
> 
> Whenever we see a READ_ONCE or other marked access it is obvious to
> the reader that there are concurrent accesses happening.  I'd argue
> that for intentional data races, we should convey similar information,
> to avoid breaking the code (of course KCSAN would tell you, but only
> after the change was done). Even moreso, since changes to code
> involving 'data_race()' will need re-verification that the data races
> are still safe.
> 
> > I really don't like it but at least there is a significant benefit from the tool
> > that probably makes it worth the visual noise.
> > 
> > Blue sky thoughts for The Far Future: It would be nice if the tools got a lot
> > better--maybe in the direction of C language extensions, even if only used in
> > this project at first.
> 
> Still thinking about this.  What we want to convey is that, while
> there are races on the particular variable, nobody should be modifying
> the bits here. Adding a READ_ONCE (or data_race()) would miss a
> harmful race where somebody modifies these bits, so in principle I
> agree. However, I think the tool can't automatically tell (even if we
> had compiler extensions to give us the bits accessed) which bits we
> care about, because we might have something like:
> 
> int foo_bar = READ_ONCE(flags) >> FOO_BAR_SHIFT;  // need the
> READ_ONCE because of FOO bits
> .. (foo_bar & FOO_MASK) ..  // FOO bits can be modified concurrently
> .. (foo_bar & BAR_MASK) ..  // nobody should modify BAR bits
> concurrently though !
> 
> What we want is to assert that nobody touches a particular set of
> bits. KCSAN has recently gotten ASSERT_EXCLUSIVE_{WRITER,ACCESS}
> macros which help assert properties of concurrent code, where bugs
> won't manifest as data races. Along those lines, I can see the value
> in doing an exclusivity check on a bitmask of a variable.
> 
> I don't know how much a READ_BITS macro could help, since it's
> probably less ergonomic to have to say something like:
>   READ_BITS(page->flags, ZONES_MASK << ZONES_PGSHIFT) >> ZONES_PGSHIFT.
> 
> 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;
>  }

Actually, it seems still need to write if I understand correctly,

ASSERT_EXCLUSIVE_BITS(page->flags, ZONES_MASK << ZONES_PGSHIFT);
return data_race((page->flags >> ZONES_PGSHIFT) & ZONES_MASK);

On the other hand, if you really worry about this thing could go wrong, it might
be better of using READ_ONCE() at the first place where it will be more future-
proof with the trade-off it might generate less efficient code optimization?

Alternatively, is there a way to write this as this?

return ASSERT_EXCLUSIVE_BITS(page->flags, ZONES_MASK << ZONES_PGSHIFT);

Kind of ugly but it probably cleaner.

> 
> 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?
> 
> Thanks,
> -- Marco
> 
> 
> 
> 
> 
> > thanks,
> > --
> > John Hubbard
> > NVIDIA
> > 
> > > > 
> > > > That's still true, but to a lesser extent if more macros are added. In this case,
> > > > I suspect that READ_BITS() makes the commenting easier and shorter. So I'd tentatively
> > > > lead towards adding it, but what do others on the list think?
> > > 
> > > Even read bits could be dangerous from data races and confusing at best, so I am not really sure what the value of introducing this new macro. People who like to understand it correctly still need to read the commit logs.
> > > 
> > > This flags->zonenum is such a special case that I don’t really see it regularly for the last few weeks digging KCSAN reports, so even if it is worth adding READ_BITS(), there are more equally important macros need to be added together to be useful initially. For example, HARMLESS_COUNTERS(), READ_SINGLE_BIT(), READ_IMMUTATABLE_BITS() etc which Linus said exactly wanted to avoid.
> > >