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

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Message-Id: <8602A57D-B420-489C-89CC-23D096014C47@lca.pw>
Date:   Sat, 8 Feb 2020 22:10:09 -0500
From:   Qian Cai <cai@....pw>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Marco Elver <elver@...gle.com>, Jan Kara <jack@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>, ira.weiny@...el.com,
        Dan Williams <dan.j.williams@...el.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH] mm: fix a data race in put_page()

> On Feb 8, 2020, at 8:44 PM, John Hubbard <jhubbard@...dia.com> wrote:
> 
> So it looks like we're probably stuck with having to annotate the code. Given
> 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 need to
> add comments for the various cases, explaining which data_race situation is 
> happening.

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

> 
> That's still true, but to a lesser extent if more macros are added. In this 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’t really see it regularly for the last few weeks digging KCSAN reports, so even if it is worth adding READ_BITS(), there are more equally important macros need to be added together to be useful initially. For example, HARMLESS_COUNTERS(), READ_SINGLE_BIT(), READ_IMMUTATABLE_BITS() etc which Linus said exactly wanted to avoid.