On Mon, Feb 04, 2013 at 08:02:43PM +0100, Oleg Nesterov wrote:
> __perf_event_exit_task() does:
>
> sync_child_event(event)
> free_event(event)
>
> sync_child_event() does put_event(event->parent) which can actually
> free ->parent. This means that event->destroy(event) is called with
> ->parent pointing to nowhere.
>
> perf_free_event() does put_parent(parent) before free_event(child)
> too.
>
> Afaics, currently this is fine. But the tracing "subclasses" (like
> trace_uprobe) may want to track the events and their parents, and even
> the fact that parent->destroy() is called before child->destroy() can
> complicate this.
>
> Move this put_event() from sync_child_event() to its single caller,
> __perf_event_exit_task(). Change perf_free_event() the same way.
looks ok to me, could prevent future headaches ;-)
jirka
>
> Signed-off-by: Oleg Nesterov <o...@redhat.com>
> ---
> kernel/events/core.c | 24 ++++++++++++------------
> 1 files changed, 12 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 301079d..1b2e516 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6787,12 +6787,6 @@ static void sync_child_event(struct perf_event
> *child_event,
> mutex_lock(&parent_event->child_mutex);
> list_del_init(&child_event->child_list);
> mutex_unlock(&parent_event->child_mutex);
> -
> - /*
> - * Release the parent event, if this was the last
> - * reference to it.
> - */
> - put_event(parent_event);
> }
>
> static void
> @@ -6800,7 +6794,9 @@ __perf_event_exit_task(struct perf_event *child_event,
> struct perf_event_context *child_ctx,
> struct task_struct *child)
> {
> - if (child_event->parent) {
> + struct perf_event *parent_event = child_event->parent;
> +
> + if (parent_event) {
> raw_spin_lock_irq(&child_ctx->lock);
> perf_group_detach(child_event);
> raw_spin_unlock_irq(&child_ctx->lock);
> @@ -6813,9 +6809,14 @@ __perf_event_exit_task(struct perf_event *child_event,
> * that are still around due to the child reference. These
> * events need to be zapped.
> */
> - if (child_event->parent) {
> + if (parent_event) {
> sync_child_event(child_event, child);
> free_event(child_event);
> + /*
> + * Release the parent event, if this was the last
> + * reference to it.
> + */
> + put_event(parent_event);
> }
> }
>
> @@ -6867,8 +6868,7 @@ static void perf_event_exit_task_context(struct
> task_struct *child, int ctxn)
> * We can recurse on the same lock type through:
> *
> * __perf_event_exit_task()
> - * sync_child_event()
> - * put_event()
> + * put_event(parent_event)
> * mutex_lock(&ctx->mutex)
> *
> * But since its the parent context it won't be the same instance.
> @@ -6937,11 +6937,11 @@ static void perf_free_event(struct perf_event *event,
> list_del_init(&event->child_list);
> mutex_unlock(&parent->child_mutex);
>
> - put_event(parent);
> -
> perf_group_detach(event);
> list_del_event(event, ctx);
> free_event(event);
> +
> + put_event(parent);
> }
>
> /*
> --
> 1.5.5.1
>
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