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

[Andrew Morton]

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

> 
> 
> > 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

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.