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

[Qian Cai]

> On Feb 9, 2020, at 9:20 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> 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,

âVarious options were considered, and based on feedback from Linus,
decided 'data_race(..)' is the best option:â

lore.kernel.org/linux-fsdevel/CAHk-=wg5CkOEF8DTez1Qu0XTEFw_oHhxN98bDnFqbY7HL5AB2g@xxxxxxxxxxxxxx/

Paul also said,

âPeople 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.â