Re: [PATCH] [RFC] perf: Fix a race between ring_buffer_detach() and ring_buffer_wakeup()

Peter Zijlstra
2014-05-07T18:06:28Z

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On Wed, May 07, 2014 at 02:35:26PM +0200, Peter Zijlstra wrote:
> static void ring_buffer_attach(struct perf_event *event,
> struct ring_buffer *rb)
> {
> + struct ring_buffer *old_rb = NULL;
> unsigned long flags;
>
> + if (event->rb) {
> + /*
> + * Should be impossible, we set this when removing
> + * event->rb_entry and wait/clear when adding event->rb_entry.
> + */
> + WARN_ON_ONCE(event->rcu_pending);
>
> + old_rb = event->rb;
> + event->rcu_batches = get_state_synchronize_rcu();
> + event->rcu_pending = 1;
>
> + spin_lock_irqsave(&rb->event_lock, flags);
> + list_del_rcu(&event->rb_entry);
> + spin_unlock_irqrestore(&rb->event_lock, flags);

This all works a whole lot better if you make that old_rb->event_lock.

> + }
>
> + if (event->rcu_pending && rb) {
> + cond_synchronize_rcu(event->rcu_batches);
> + event->rcu_pending = 0;
> + }
>
> + if (rb) {
> + spin_lock_irqsave(&rb->event_lock, flags);
> + list_add_rcu(&event->rb_entry, &rb->event_list);
> + spin_unlock_irqrestore(&rb->event_lock, flags);
> + }
> +
> + rcu_assign_pointer(event->rb, rb);
> +
> + if (old_rb) {
> + ring_buffer_put(old_rb);
> + /*
> + * Since we detached before setting the new rb, so that we
> + * could attach the new rb, we could have missed a wakeup.
> + * Provide it now.
> + */
> + wake_up_all(&event->waitq);
> + }
> }

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