[PATCH 1/1] mm/ksm: avoid missing ksmd wakeups in ksm_enter

[Longlong Xia]

From: Longlong Xia <xialonglong@xxxxxxxxxx>

__ksm_enter() decides whether ksmd needs a wakeup by checking if the
mm slot list is empty before inserting the new slot.

The empty check is currently outside ksm_mmlist_lock.  Another CPU can
remove the last slot and let ksmd go back to sleep after the unlocked
check, while this CPU inserts a new slot and skips the wakeup based on
the stale result.

Take ksm_mmlist_lock before checking the list so the empty-to-nonempty
transition and the insertion are observed as one critical section.

Fixes: 6e15838425ac ("ksm: keep quiet while list empty")
Signed-off-by: Longlong Xia <xialonglong@xxxxxxxxxx>
---
 mm/ksm.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 7d5b76478f0b..3f2f4707ca9e 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -3019,7 +3019,7 @@ int __ksm_enter(struct mm_struct *mm)
 {
 	struct ksm_mm_slot *mm_slot;
 	struct mm_slot *slot;
-	int needs_wakeup;
+	bool needs_wakeup;
 
 	mm_slot = mm_slot_alloc(mm_slot_cache);
 	if (!mm_slot)
@@ -3027,10 +3027,9 @@ int __ksm_enter(struct mm_struct *mm)
 
 	slot = &mm_slot->slot;
 
+	spin_lock(&ksm_mmlist_lock);
 	/* Check ksm_run too?  Would need tighter locking */
 	needs_wakeup = list_empty(&ksm_mm_head.slot.mm_node);
-
-	spin_lock(&ksm_mmlist_lock);
 	mm_slot_insert(mm_slots_hash, mm, slot);
 	/*
 	 * When KSM_RUN_MERGE (or KSM_RUN_STOP),
-- 
2.43.0