On Mon, Feb 18, 2013 at 02:12:23PM +0800, Li Zefan wrote:
> commit 205a872bd6f9a9a09ef035ef1e90185a8245cc58 ("cgroup: fix lockdep
> warning for event_control") solved a deadlock by introducing a new
> bug.
>
> Move cgrp->event_list to a temporary list doesn't mean you can traverse
> this list locklessly, because at the same time cgroup_event_wake() can
> be called and remove the event from the list. The result of this race
> is disastrous.
>
> We adopt the way how kvm irqfd code implements race-free event removal,
> which is now described in the comments in cgroup_event_wake().
>
> Signed-off-by: Li Zefan <lizefan@xxxxxxxxxx>
> ---
> kernel/cgroup.c | 50 ++++++++++++++++++++++++++++++++++----------------
> 1 file changed, 34 insertions(+), 16 deletions(-)
>
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 26c071c..65c8101 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -217,6 +217,10 @@ struct cgroup_event {
> */
> struct list_head list;
> /*
> + * Need to notify userspace when this event is removed?
> + */
> + bool signal_on_remove;
> + /*
> * All fields below needed to unregister event when
> * userspace closes eventfd.
> */
> @@ -3833,8 +3837,17 @@ static void cgroup_event_remove(struct work_struct *work)
> remove);
> struct cgroup *cgrp = event->cgrp;
>
> + remove_wait_queue(event->wqh, &event->wait);
> +
> event->cft->unregister_event(cgrp, event->cft, event->eventfd);
>
> + /*
> + * If this event is to be removed due to cgroup removal,
> + * we notify userspace.
> + */
> + if (event->signal_on_remove)
> + eventfd_signal(event->eventfd, 1);
It's safe to notify anyway, isn't it? Let's just drop signal_on_remove.
Otherwise, look good.
Acked-by: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
--
Kirill A. Shutemov
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