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

[Qian Cai]

> On Feb 25, 2020, at 9:11 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
> On Tue, 25 Feb 2020 11:55:26 -0500 Qian Cai <cai@xxxxxx> 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). The data races were reported by KCSAN,
>> 
>> ...
>> 
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -3961,11 +3961,10 @@ void wakeup_kswapd(struct zone *zone, gfp_t gfp_flags, int order,
>> 		return;
>> 	pgdat = zone->zone_pgdat;
>> 
>> -	if (pgdat->kswapd_classzone_idx == MAX_NR_ZONES)
>> -		pgdat->kswapd_classzone_idx = classzone_idx;
>> -	else
>> -		pgdat->kswapd_classzone_idx = max(pgdat->kswapd_classzone_idx,
>> -						  classzone_idx);
>> +	if (READ_ONCE(pgdat->kswapd_classzone_idx) == MAX_NR_ZONES ||
>> +	    READ_ONCE(pgdat->kswapd_classzone_idx) < classzone_idx)
>> +		WRITE_ONCE(pgdat->kswapd_classzone_idx, classzone_idx);
>> +
>> 	pgdat->kswapd_order = max(pgdat->kswapd_order, order);
>> 	if (!waitqueue_active(&pgdat->kswapd_wait))
>> 		return;
> 
> This is very partial, isn't it?  The above code itself is racy against
> other code which manipulates ->kswapd_classzone_idx and the
> manipulation in allow_direct_reclaim() is performed by threads other
> than kswapd and so need the READ_ONCE treatment and is still racy with
> that?

Right, I suppose allow_direct_reclaim() could use some treatment too.

> 
> I guess occasional races here don't really matter, but a grossly wrong
> read from load tearing might matter.  In which case shouldn't we be
> defending against them in all cases where non-kswapd threads read this
> field?