linux-kernel - Re: [PATCH] mm: fix a data race in put

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Date:   Thu, 6 Feb 2020 19:18:16 -0500
From:   Qian Cai <cai@....pw>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Jan Kara <jack@...e.cz>, David Hildenbrand <david@...hat.com>,
        akpm@...ux-foundation.org, ira.weiny@...el.com,
        dan.j.williams@...el.com, elver@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: fix a data race in put_page()



> On Feb 6, 2020, at 6:34 PM, John Hubbard <jhubbard@...dia.com> wrote:
> 
> On 2/6/20 7:23 AM, Qian Cai wrote:
>> 
>> 
>>> On Feb 6, 2020, at 9:55 AM, Jan Kara <jack@...e.cz> wrote:
>>> 
>>> I don't think the problem is real. The question is how to make KCSAN happy
>>> in a way that doesn't silence other possibly useful things it can find and
>>> also which makes it most obvious to the reader what's going on... IMHO
>>> using READ_ONCE() fulfills these targets nicely - it is free
>>> performance-wise in this case, it silences the checker without impacting
>>> other races on page->flags, its kind of obvious we don't want the load torn
>>> in this case so it makes sense to the reader (although a comment may be
>>> nice).
>> 
>> Actually, use the data_race() macro there fulfilling the same purpose too, i.e, silence the splat here but still keep searching for other races.
>> 
> 
> Yes, but both READ_ONCE() and data_race() would be saying untrue things about this code,
> and that somewhat offends my sense of perfection... :)
> 
> * READ_ONCE(): this field need not be restricted to being read only once, so the
>  name is immediately wrong. We're using side effects of READ_ONCE().
> 
> * data_race(): there is no race on the N bits worth of page zone number data. There
>  is only a perceived race, due to tools that look at word-level granularity.
> 
> I'd propose one or both of the following:
> 
> a) Hope that Marcus has an idea to enhance KCSAN so as to support this model of
>   access, and/or

A similar thing was brought up before, i.e., anything compared to zero is immune to load-tearing
issues, but it is rather difficult to implement it in the compiler, so it was settled to use data_race(),

https://lore.kernel.org/lkml/CANpmjNN8J1oWtLPHTgCwbbtTuU_Js-8HD=cozW5cYkm8h-GTBg@mail.gmail.com/#r

> 
> b) Add a new, better-named macro to indicate what's going on. Initial bikeshed-able
>   candidates:
> 
> 	READ_RO_BITS()
> 	READ_IMMUTABLE_BITS()
> 	...etc...
> 

Actually, Linus might hate those kinds of complication rather than a simple data_race() macro,

https://lore.kernel.org/linux-fsdevel/CAHk-=wg5CkOEF8DTez1Qu0XTEFw_oHhxN98bDnFqbY7HL5AB2g@mail.gmail.com/