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From:   Qian Cai <cai@lca.pw>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH] mm: fix a data race in put_page()
Date:   Sat, 8 Feb 2020 22:10:09 -0500
Message-Id: <8602A57D-B420-489C-89CC-23D096014C47@lca.pw>
References: <5402183a-2372-b442-84d3-c28fb59fa7af@nvidia.com>
Cc:     Marco Elver <elver@google.com>, 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>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>
In-Reply-To: <5402183a-2372-b442-84d3-c28fb59fa7af@nvidia.com>
To:     John Hubbard <jhubbard@nvidia.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Feb 8, 2020, at 8:44 PM, John Hubbard <jhubbard@nvidia.com> wrote:
>=20
> So it looks like we're probably stuck with having to annotate the code. Gi=
ven
> that, there is a balance between how many macros, and how much commenting.=
 For
> example, if there is a single macro (data_race, for example), then we'll n=
eed to
> add comments for the various cases, explaining which data_race situation i=
s=20
> happening.

On the other hand, it is perfect fine of not commenting on each data_race() t=
hat most of times, people could run git blame to learn more details. Actuall=
y, no maintainers from various of subsystems asked for commenting so far.

>=20
> That's still true, but to a lesser extent if more macros are added. In thi=
s case,
> I suspect that READ_BITS() makes the commenting easier and shorter. So I'd=
 tentatively
> lead towards adding it, but what do others on the list think?

Even read bits could be dangerous from data races and confusing at best, so I=
 am not really sure what the value of introducing this new macro. People who=
 like to understand it correctly still need to read the commit logs.

This flags->zonenum is such a special case that I don=E2=80=99t really see i=
t regularly for the last few weeks digging KCSAN reports, so even if it is w=
orth adding READ_BITS(), there are more equally important macros need to be a=
dded together to be useful initially. For example, HARMLESS_COUNTERS(), READ=
_SINGLE_BIT(), READ_IMMUTATABLE_BITS() etc which Linus said exactly wanted t=
o avoid.=