From: Qian Cai <cai@lca.pw>
To: John Hubbard <jhubbard@nvidia.com>
Cc: Jan Kara <jack@suse.cz>, David Hildenbrand <david@redhat.com>,
akpm@linux-foundation.org, ira.weiny@intel.com,
dan.j.williams@intel.com, elver@google.com, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: fix a data race in put_page()
Date: Thu, 6 Feb 2020 19:18:16 -0500 [thread overview]
Message-ID: <235ACF21-35BE-4EDA-BA64-9553DA53BF12@lca.pw> (raw)
In-Reply-To: <079c4429-8a11-154d-cf5c-473d2698d18d@nvidia.com>
> On Feb 6, 2020, at 6:34 PM, John Hubbard <jhubbard@nvidia.com> wrote:
>
> On 2/6/20 7:23 AM, Qian Cai wrote:
>>
>>
>>> On Feb 6, 2020, at 9:55 AM, Jan Kara <jack@suse.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/
next prev parent reply other threads:[~2020-02-07 0:18 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-06 13:17 [PATCH] mm: fix a data race in put_page() Qian Cai
2020-02-06 13:33 ` David Hildenbrand
2020-02-06 13:51 ` Qian Cai
2020-02-06 14:55 ` Jan Kara
2020-02-06 14:59 ` David Hildenbrand
2020-02-06 15:23 ` Qian Cai
2020-02-06 23:34 ` John Hubbard
2020-02-06 23:36 ` John Hubbard
2020-02-06 23:55 ` Qian Cai
2020-02-07 0:18 ` Qian Cai [this message]
2020-02-07 0:27 ` John Hubbard
2020-02-07 0:55 ` Qian Cai
2020-02-07 13:17 ` Marco Elver
2020-02-09 1:44 ` John Hubbard
2020-02-09 3:10 ` Qian Cai
2020-02-09 7:12 ` John Hubbard
2020-02-10 7:48 ` Marco Elver
2020-02-10 12:16 ` Qian Cai
2020-02-10 12:58 ` Marco Elver
2020-02-10 13:36 ` Qian Cai
2020-02-10 13:38 ` Marco Elver
2020-02-10 13:55 ` Qian Cai
2020-02-10 14:12 ` Marco Elver
2020-02-10 14:31 ` Qian Cai
2020-02-10 16:23 ` Qian Cai
2020-02-10 16:33 ` Marco Elver
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