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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, May 07, 2014 at 08:06:21PM +0200, Peter Zijlstra wrote:
> 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);

There is not a whole lot of code between the get_state_synchronize_rcu()
and the cond_synchronize_rcu(), so I would expect this to do a
synchronize_rcu() almost all the time.  Or am I missing something here?

							Thanx, Paul

> > +		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);
> > +	}
> >  }