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Subject: Re: [PATCH] mm: fix a data race in put_page()
From:   Qian Cai <cai@lca.pw>
In-Reply-To: <90ab0b09-0f70-fe6d-259e-f529f4ef9174@nvidia.com>
Date:   Thu, 6 Feb 2020 19:55:38 -0500
Cc:     Jan Kara <jack@suse.cz>, David Hildenbrand <david@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>, ira.weiny@intel.com,
        Dan Williams <dan.j.williams@intel.com>,
        Marco Elver <elver@google.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Message-Id: <1CFC5A47-3334-4696-89FE-CDF01026B12B@lca.pw>
References: <20200206145501.GD26114@quack2.suse.cz>
 <D022CBB0-C8EC-4F5A-A0B0-893AA7A014AA@lca.pw>
 <079c4429-8a11-154d-cf5c-473d2698d18d@nvidia.com>
 <235ACF21-35BE-4EDA-BA64-9553DA53BF12@lca.pw>
 <90ab0b09-0f70-fe6d-259e-f529f4ef9174@nvidia.com>
To:     John Hubbard <jhubbard@nvidia.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Feb 6, 2020, at 7:27 PM, John Hubbard <jhubbard@nvidia.com> wrote:
>=20
> On 2/6/20 4:18 PM, Qian Cai wrote:
>>> 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).
>>>>=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 Marco (I've fixed the typo in his name. --jh) has an =
idea to enhance KCSAN so as to support this model of
>>>  access, and/or
>>=20
>> 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(),
>>=20
>> =
https://lore.kernel.org/lkml/CANpmjNN8J1oWtLPHTgCwbbtTuU_Js-8HD=3DcozW5cYk=
m8h-GTBg@mail.gmail.com/#r
>>=20
>=20
>=20
> Thanks for that link to the previous discussion, good context.
>=20
>=20
>>>=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
>>=20
>> Actually, Linus might hate those kinds of complication rather than a =
simple data_race() macro,
>>=20
>> =
https://lore.kernel.org/linux-fsdevel/CAHk-=3Dwg5CkOEF8DTez1Qu0XTEFw_oHhxN=
98bDnFqbY7HL5AB2g@mail.gmail.com/
>>=20
>=20
> Another good link. However, my macros above haven't been proposed yet, =
and I'm perfectly=20
> comfortable proposing something that Linus *might* (or might not!) =
hate. No point in=20
> guessing about it, IMHO.
>=20
> If you want, I'll be happy to put on my flame suit and post a patchset =
proposing=20
> READ_IMMUTABLE_BITS() (or a better-named thing, if someone has another =
name idea).  :)
>=20

BTW, the current comment said (note, it is called =E2=80=9Caccess=E2=80=9D=
 which in this case it does read the whole word
rather than those 3 bits, even though it is only those bits are of =
interested for us),

/*
 * 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.
 */

Macro might have more to say.=