linux-kernel - Re: [PATCH -next v2] mm: mark an intentional data race in page

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Date:   Thu, 6 Feb 2020 15:25:41 -0500
From:   Qian Cai <cai@....pw>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Marco Elver <elver@...gle.com>, jhubbard@...dia.com,
        ira.weiny@...el.com, dan.j.williams@...el.com, jack@...e.cz,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next v2] mm: mark an intentional data race in
 page_zonenum



> On Feb 6, 2020, at 3:20 PM, Matthew Wilcox <willy@...radead.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.