Re: [PATCH] mm/page_counter: fix various data races

[Tetsuo Handa]

On 2020/01/29 21:21, Marco Elver wrote:
>> By the way, can READ_ONCE()/WRITE_ONCE() really solve this warning?
>> The link above says read/write on the same location ( mm/page_counter.c:129 ).
>> I don't know how READ_ONCE()/WRITE_ONCE() can solve the race.
> 
> It avoids the *data* race, with *_ONCE telling the compiler to not
> optimize the accesses in concurrency-unfriendly ways.  Since *_ONCE is
> used, it conveys clear intent that the code here is meant to be
> concurrent, and KCSAN stops complaining (and assumes that the *logic*
> is correct).

I see. Unlike c->failcnt++ which involves read-modify-write, *_ONCE() can be used for
simple read (like c->watermark) or simple write (like c->watermark = new) case.