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SubjectRe: [PATCH 2/2] mm: slince possible data races about pgdat->kswapd
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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);
>> }
>> }
>>
>

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