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Message-Id: <2B333FA6-AB17-4169-B9EE-9355FF9C42A4@lca.pw>
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.”
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