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

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

On Fri, Mar 07, 2014 at 03:38:46PM +0200, Alexander Shishkin wrote:
> This is more of a problem description than an actual bugfix, but currently
> ring_buffer_detach() can kick in while ring_buffer_wakeup() is traversing
> the ring buffer's event list, leading to cpu stalls.
> 
> What this patch does is crude, but fixes the problem, which is: one rcu
> grace period has to elapse between ring_buffer_detach() and subsequent
> ring_buffer_attach(), otherwise either the attach will fail or the wakeup
> will misbehave. Also, making it a call_rcu() callback will make it race
> with attach().
> 
> Another solution that I see is to check for list_empty(&event->rb_entry)
> before wake_up_all() in ring_buffer_wakeup() and restart the list
> traversal if it is indeed empty, but that is ugly too as there will be
> extra wakeups on some events.
> 
> Anything that I'm missing here? Any better ideas?

Not sure it qualifies as "better", but git call to ring_buffer_detach()
is going to free the event anyway, so the synchronize_rcu() and the
INIT_LIST_HEAD() should not be needed in that case.  I am guessing that
the same is true for perf_mmap_close().

So that leaves the call in perf_event_set_output(), which detaches from an
old rb before attaching that same event to a new one.  So maybe have the
synchronize_rcu() and INIT_LIST_HEAD() instead be in the "if (old_rb)",
which might be a reasonably uncommon case?

							Thanx, Paul

> Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
> ---
>  kernel/events/core.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 661951a..bce41e0 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3861,7 +3861,7 @@ static void ring_buffer_attach(struct perf_event *event,
> 
>  	spin_lock_irqsave(&rb->event_lock, flags);
>  	if (list_empty(&event->rb_entry))
> -		list_add(&event->rb_entry, &rb->event_list);
> +		list_add_rcu(&event->rb_entry, &rb->event_list);
>  	spin_unlock_irqrestore(&rb->event_lock, flags);
>  }
> 
> @@ -3873,9 +3873,11 @@ static void ring_buffer_detach(struct perf_event *event, struct ring_buffer *rb)
>  		return;
> 
>  	spin_lock_irqsave(&rb->event_lock, flags);
> -	list_del_init(&event->rb_entry);
> +	list_del_rcu(&event->rb_entry);
>  	wake_up_all(&event->waitq);
>  	spin_unlock_irqrestore(&rb->event_lock, flags);
> +	synchronize_rcu();
> +	INIT_LIST_HEAD(&event->rb_entry);
>  }
> 
>  static void ring_buffer_wakeup(struct perf_event *event)
> -- 
> 1.9.0
>