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

From: John Hubbard <jhubbard@nvidia.com>
To: Qian Cai <cai@lca.pw>, Jan Kara <jack@suse.cz>
Cc: David Hildenbrand <david@redhat.com>, <akpm@linux-foundation.org>,
	<ira.weiny@intel.com>, <dan.j.williams@intel.com>,
	<elver@google.com>, <linux-mm@kvack.org>,
	<linux-kernel@vger.kernel.org>, Marco Elver <elver@google.com>
Subject: Re: [PATCH] mm: fix a data race in put_page()
Date: Thu, 6 Feb 2020 15:36:37 -0800	[thread overview]
Message-ID: <1978a849-1db9-2d8a-d494-978eadc7d999@nvidia.com> (raw)
In-Reply-To: <079c4429-8a11-154d-cf5c-473d2698d18d@nvidia.com>

On 2/6/20 3:34 PM, John Hubbard 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).
>>
>> 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.
>>
> 
> Yes, but both READ_ONCE() and data_race() would be saying untrue things about this code,
> and that somewhat offends my sense of perfection... :)
> 
> * 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().
> 
> * 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.
> 
> I'd propose one or both of the following:
> 
> a) Hope that Marcus has an idea to enhance KCSAN so as to support this model of
>    access, and/or
> 

arggh, make that "Marco Elver"! Really sorry for the typo on your name. 

thanks,
-- 
John Hubbard
NVIDIA

> b) Add a new, better-named macro to indicate what's going on. Initial bikeshed-able
>    candidates:
> 
> 	READ_RO_BITS()
> 	READ_IMMUTABLE_BITS()
> 	...etc...
> 
> thanks,
> 