linux-kernel - Re: [PATCH -next v2] mm: mark an intentional data race in page

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Date:   Sun, 9 Feb 2020 20:06:20 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Qian Cai <cai@....pw>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Marco Elver <elver@...gle.com>, jhubbard@...dia.com,
        ira.weiny@...el.com, dan.j.williams@...el.com, jack@...e.cz,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next v2] mm: mark an intentional data race in
 page_zonenum

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

> 
> 
> > On Feb 9, 2020, at 9:20 PM, Andrew Morton <akpm@...ux-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@...l.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.