[PATCH] ksm: fix race between ksmd and exiting task

[Chris Wright <chrisw@sous-sol.org>]

Andrea Righi reported a case where an exiting task can race against
ksmd.

ksm_scan.mm_slot == the only registered mm
CPU 1 (bug program)		CPU 2 (ksmd)
 				list_empty() is false
lock
ksm_scan.mm_slot
list_del
unlock
 				slot == &ksm_mm_head (but list is now empty_)

Close this race by revalidating that the new slot is not simply the list
head again.

Reported-by: Andrea Righi <andrea@betterlinux.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
---
 mm/ksm.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 942dfc7..0373ce4 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1301,6 +1301,9 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page)
 		slot = list_entry(slot->mm_list.next, struct mm_slot, mm_list);
 		ksm_scan.mm_slot = slot;
 		spin_unlock(&ksm_mmlist_lock);
+		/* We raced against exit of last slot on the list */
+		if (slot == &ksm_mm_head)
+			return NULL;
 next_mm:
 		ksm_scan.address = 0;
 		ksm_scan.rmap_list = &slot->rmap_list;

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