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

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Message-ID: <079c4429-8a11-154d-cf5c-473d2698d18d@nvidia.com>
Date:   Thu, 6 Feb 2020 15:34:32 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Qian Cai <cai@....pw>, Jan Kara <jack@...e.cz>
CC:     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 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

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

	READ_RO_BITS()
	READ_IMMUTABLE_BITS()
	...etc...

thanks,
-- 
John Hubbard
NVIDIA