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

[Peter Zijlstra]

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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