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.