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

[Kefeng Wang]

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.

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);
 	}
 }
 
-- 
2.35.3