Re: [PATCH -next v2] mm: mark an intentional data race in page_zonenum

[Qian Cai <cai@lca.pw>]

> On Feb 6, 2020, at 3:20 PM, Matthew Wilcox <willy@infradead.org> wrote:
> 
> On Thu, Feb 06, 2020 at 01:30:00PM -0500, Qian Cai wrote:
>> Both the read and write are done only with the non-exclusive mmap_sem
>> held. Since the read only check for a specific bit range (up to 3 bits)
>> in the flag but the write here never change those 3 bits, so load
>> tearing would be harmless here. Thus, just mark it as an intentional
>> data races using the data_race() macro which is designed for those
>> situations [1].
> 
> This changelog makes me think you don't really understand the situation.
> 
> A page never changes its zone number.  The zone number happens to be
> stored in the same word as other bits which are modified, but the zone
> number bits will never be modified by any other write.  So we can accept
> a reload of the zone bits after an intervening write and we don't need
> to use READ_ONCE().

Maybe your explanation is better, but I did try to explain the same thing.
I’ll let Andrew to decide if he would like to update the commit log a bit
with your wording.