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From: Qian Cai <cai@lca.pw>
To: John Hubbard <jhubbard@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>
Subject: Re: [PATCH] mm: fix a data race in put_page()
Date: Sat, 8 Feb 2020 22:10:09 -0500	[thread overview]
Message-ID: <8602A57D-B420-489C-89CC-23D096014C47@lca.pw> (raw)
In-Reply-To: <5402183a-2372-b442-84d3-c28fb59fa7af@nvidia.com>



> On Feb 8, 2020, at 8:44 PM, John Hubbard <jhubbard@nvidia.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.

  reply	other threads:[~2020-02-09  3:10 UTC|newest]

Thread overview: 34+ 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 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
2020-02-07  0:27           ` John Hubbard
2020-02-07  0:55             ` Qian Cai
2020-02-07 13:17               ` Marco Elver
2020-02-07 13:17                 ` Marco Elver
2020-02-09  1:44                 ` John Hubbard
2020-02-09  3:10                   ` Qian Cai [this message]
2020-02-09  7:12                     ` John Hubbard
2020-02-10  7:48                       ` Marco Elver
2020-02-10  7:48                         ` Marco Elver
2020-02-10 12:16                         ` Qian Cai
2020-02-10 12:58                           ` Marco Elver
2020-02-10 12:58                             ` Marco Elver
2020-02-10 13:36                             ` Qian Cai
2020-02-10 13:38                               ` Marco Elver
2020-02-10 13:38                                 ` Marco Elver
2020-02-10 13:55                                 ` Qian Cai
2020-02-10 14:12                                   ` Marco Elver
2020-02-10 14:12                                     ` Marco Elver
2020-02-10 14:31                                     ` Qian Cai
2020-02-10 16:23                         ` Qian Cai
2020-02-10 16:23                           ` Qian Cai
2020-02-10 16:33                           ` Marco Elver
2020-02-10 16:33                             ` Marco Elver

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