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

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Date:   Sun, 9 Feb 2020 21:41:56 -0500
From:   Qian Cai <cai@....pw>
To:     Andrew Morton <akpm@...ux-foundation.org>
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 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.”