Re: [PATCH] mm: fix a data race in put_page()

From: John Hubbard <jhubbard@nvidia.com>
To: Qian Cai <cai@lca.pw>
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 23:12:48 -0800	[thread overview]
Message-ID: <1a179bea-fd71-7b53-34c5-895986c24931@nvidia.com> (raw)
In-Reply-To: <8602A57D-B420-489C-89CC-23D096014C47@lca.pw>

On 2/8/20 7:10 PM, Qian Cai wrote:
> 
> 
>> 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.
> 

Well, maybe I'm looking at this wrong. I was thinking that one should attempt to
understand the code on the screen, and that's generally best--but here, maybe
"data_race" is just something that means "tool cruft", really. So mentally we
would move toward visually filtering out the data_race "key word".

I really don't like it but at least there is a significant benefit from the tool
that probably makes it worth the visual noise.

Blue sky thoughts for The Far Future: It would be nice if the tools got a lot
better--maybe in the direction of C language extensions, even if only used in
this project at first.

thanks,
-- 
John Hubbard
NVIDIA

>>
>> 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.
> 