Re: [PATCH 2/2] mm: slince possible data races about pgdat->kswapd

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2022/8/24 16:24, David Hildenbrand wrote:
> On 24.08.22 09:19, Kefeng Wang wrote:
>> The pgdat->kswapd could be accessed concurrently by kswapd_run() and
>> kcompactd(), it don't be protected by any lock, which could leads to
>> data races, adding READ/WRITE_ONCE() to slince it.
> Okay, I think this patch here makes it clearer that we really just want
> proper synchronization instead of hacking around it.
>
> What speaks against protecting pgdat->kswapd this using some proper
> locking primitive?

So add a new lock into struct pglist_data to protect pgdat->kswapd，other

option, thanks.

>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>> ---
>>   mm/compaction.c | 4 +++-
>>   mm/vmscan.c     | 8 ++++----
>>   2 files changed, 7 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 640fa76228dd..aa1cfe47f046 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -1983,7 +1983,9 @@ static inline bool is_via_compact_memory(int order)
>>   
>>   static bool kswapd_is_running(pg_data_t *pgdat)
>>   {
>> -	return pgdat->kswapd && task_is_running(pgdat->kswapd);
>> +	struct task_struct *t = READ_ONCE(pgdat->kswapd);
>> +
>> +	return t && task_is_running(t);
>>   }
>>   
>>   /*
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 08c6497f76c3..65b19ca8c8ee 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -4644,7 +4644,7 @@ void kswapd_run(int nid)
>>   	pg_data_t *pgdat = NODE_DATA(nid);
>>   	struct task_struct *t;
>>   
>> -	if (pgdat->kswapd)
>> +	if (READ_ONCE(pgdat->kswapd))
>>   		return;
>>   
>>   	t = kthread_run(kswapd, pgdat, "kswapd%d", nid);
>> @@ -4653,7 +4653,7 @@ void kswapd_run(int nid)
>>   		BUG_ON(system_state < SYSTEM_RUNNING);
>>   		pr_err("Failed to start kswapd on node %d\n", nid);
>>   	} else {
>> -		pgdat->kswapd = t;
>> +		WRITE_ONCE(pgdat->kswapd, t);
>>   	}
>>   }
>>   
>> @@ -4663,11 +4663,11 @@ void kswapd_run(int nid)
>>    */
>>   void kswapd_stop(int nid)
>>   {
>> -	struct task_struct *kswapd = NODE_DATA(nid)->kswapd;
>> +	struct task_struct *kswapd = READ_ONCE(NODE_DATA(nid)->kswapd);
>>   
>>   	if (kswapd) {
>>   		kthread_stop(kswapd);
>> -		NODE_DATA(nid)->kswapd = NULL;
>> +		WRITE_ONCE(NODE_DATA(nid)->kswapd, NULL);
>>   	}
>>   }
>>   
>