Re: [PATCH -next v2] mm: mark an intentional data race in page_zonenum

Andrew Morton
2020-02-10T04:06:23Z

On Sun, 9 Feb 2020 21:41:56 -0500 Qian Cai <cai@lca.pw> wrote:

>
>
> > On Feb 9, 2020, at 9:20 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> > Using data_race() here seems misleading - there is no race, but we're
> > using data_race() to suppress a false positive warning from KCSAN, yes?
>
> It is a data race in the sense of compilers, i.e., KCSAN is a compiler instrumentation, so here the load and store are both in word-size, but code here is only interested in 3 bits which are never changed. Thus, it is a harmless data race.
>
> Marco also mentioned,
>
> “Various options were considered, and based on feedback from Linus,
> decided 'data_race(..)' is the best option:”
>
> lore.kernel.org/linux-fsdevel/CAHk-=wg5CkOEF8DTez1Qu0XTEFw_oHhxN98bDnFqbY7HL5AB2g@mail.gmail.com/
>
> Paul also said,
>
> ”People will get used to the name more quickly than they will get used
> to typing the extra seven characters. Here is the current comment header:
>
> /*
> * data_race(): macro to document that accesses in an expression may conflict with
> * other concurrent accesses resulting in data races, but the resulting
> * behaviour is deemed safe regardless.
> *
> * This macro *does not* affect normal code generation, but is a hint to tooling
> * that data races here should be ignored.
> */
>
> I will be converting this to docbook form.
>
> In addition, in the KCSAN documentation:
>
> * KCSAN understands the ``data_race(expr)`` annotation, which tells KCSAN that
> any data races due to accesses in ``expr`` should be ignored and resulting
> behaviour when encountering a data race is deemed safe.”

OK. But I believe page_zonenum() still deserves a comment explaining
that there is no race and explaining why we're using data_race()
anyway. Otherwise the use of data_race() is simply misleading.