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

[David Hildenbrand <david@redhat.com>]

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?

> 
> 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);
>  	}
>  }
>  


-- 
Thanks,

David / dhildenb