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@xxxxxxxxxx>
> ---
> 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
