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SubjectRe: [PATCH] ksm: fix race between ksmd and exiting task
On Thu, 2 Jun 2011, Chris Wright wrote:

> 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.

Remarkably similar to my patch: it must be good!
But yours appears to be more popular - thanks, Chris.

>
> Reported-by: Andrea Righi <andrea@betterlinux.com>
> Cc: Hugh Dickins <hughd@google.com>

Acked-by: Hugh Dickins <hughd@google.com>

> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Signed-off-by: Chris Wright <chrisw@sous-sol.org>

Cc: stable@kernel.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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