linux-kernel - Re: [PATCH v2] mm/vmscan: fix data races at kswapd_classzone

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Date:   Tue, 25 Feb 2020 20:06:12 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Qian Cai <cai@....pw>
Cc:     akpm@...ux-foundation.org, elver@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/vmscan: fix data races at kswapd_classzone_idx

On Tue, Feb 25, 2020 at 10:58:27PM -0500, Qian Cai wrote:
> pgdat->kswapd_classzone_idx could be accessed concurrently in
> wakeup_kswapd(). Plain writes and reads without any lock protection
> result in data races. Fix them by adding a pair of READ|WRITE_ONCE() as
> well as saving a branch (compilers might well optimize the original code
> in an unintentional way anyway). While at it, also take care of
> pgdat->kswapd_order and non-kswapd threads in allow_direct_reclaim().

I don't understand why the usages of kswapd_classzone_idx in kswapd() and
kswapd_try_to_sleep() don't need changing too?  kswapd_classzone_idx()
looks safe to me, but I'm prone to missing stupid things that compilers
are allowed to do.