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Subject: Re: [PATCH] mm: fix a data race in put_page()
From:   Qian Cai <cai@lca.pw>
In-Reply-To: <079c4429-8a11-154d-cf5c-473d2698d18d@nvidia.com>
Date:   Thu, 6 Feb 2020 19:18:16 -0500
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
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Message-Id: <235ACF21-35BE-4EDA-BA64-9553DA53BF12@lca.pw>
References: <20200206145501.GD26114@quack2.suse.cz>
 <D022CBB0-C8EC-4F5A-A0B0-893AA7A014AA@lca.pw>
 <079c4429-8a11-154d-cf5c-473d2698d18d@nvidia.com>
To:     John Hubbard <jhubbard@nvidia.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Feb 6, 2020, at 6:34 PM, John Hubbard <jhubbard@nvidia.com> wrote:
>=20
> On 2/6/20 7:23 AM, Qian Cai wrote:
>>=20
>>=20
>>> On Feb 6, 2020, at 9:55 AM, Jan Kara <jack@suse.cz> wrote:
>>>=20
>>> 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).
>>=20
>> 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.
>>=20
>=20
> Yes, but both READ_ONCE() and data_race() would be saying untrue =
things about this code,
> and that somewhat offends my sense of perfection... :)
>=20
> * 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().
>=20
> * 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.
>=20
> I'd propose one or both of the following:
>=20
> 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=3DcozW5cYk=
m8h-GTBg@mail.gmail.com/#r

>=20
> b) Add a new, better-named macro to indicate what's going on. Initial =
bikeshed-able
>   candidates:
>=20
> 	READ_RO_BITS()
> 	READ_IMMUTABLE_BITS()
> 	...etc...
>=20

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

=
https://lore.kernel.org/linux-fsdevel/CAHk-=3Dwg5CkOEF8DTez1Qu0XTEFw_oHhxN=
98bDnFqbY7HL5AB2g@mail.gmail.com/