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

[John Hubbard]

On 2/6/20 4:18 PM, Qian Cai wrote:
>> On Feb 6, 2020, at 6:34 PM, John Hubbard <jhubbard@xxxxxxxxxx> wrote:
>> On 2/6/20 7:23 AM, Qian Cai wrote:
>>>> On Feb 6, 2020, at 9:55 AM, Jan Kara <jack@xxxxxxx> 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 Marco (I've fixed the typo in his name. --jh) has an idea to enhance KCSAN so as to support this model of
>>   access, and/or
> 
> A similar thing was brought up before, i.e., anything compared to zero is immune to load-tearing
> issues, but it is rather difficult to implement it in the compiler, so it was settled to use data_race(),
> 
> https://lore.kernel.org/lkml/CANpmjNN8J1oWtLPHTgCwbbtTuU_Js-8HD=cozW5cYkm8h-GTBg@xxxxxxxxxxxxxx/#r
> 


Thanks for that link to the previous discussion, good context.


>>
>> b) Add a new, better-named macro to indicate what's going on. Initial bikeshed-able
>>   candidates:
>>
>> 	READ_RO_BITS()
>> 	READ_IMMUTABLE_BITS()
>> 	...etc...
>>
> 
> Actually, Linus might hate those kinds of complication rather than a simple data_race() macro,
> 
> https://lore.kernel.org/linux-fsdevel/CAHk-=wg5CkOEF8DTez1Qu0XTEFw_oHhxN98bDnFqbY7HL5AB2g@xxxxxxxxxxxxxx/
> 

Another good link. However, my macros above haven't been proposed yet, and I'm perfectly 
comfortable proposing something that Linus *might* (or might not!) hate. No point in 
guessing about it, IMHO.

If you want, I'll be happy to put on my flame suit and post a patchset proposing 
READ_IMMUTABLE_BITS() (or a better-named thing, if someone has another name idea).  :)



thanks,
-- 
John Hubbard
NVIDIA