Re: [PATCH v2] mm/vmscan: fix data races at kswapd_classzone_idx

[Matthew Wilcox]

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.