Re: [PATCH] tracing: add trace_event_read_lock()

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

On Tue, May 19, 2009 at 01:15:44PM +0800, Lai Jiangshan wrote:
> Paul E. McKenney wrote:
> > On Mon, May 18, 2009 at 03:59:31PM +0200, Frederic Weisbecker wrote:
> >> On Mon, May 18, 2009 at 07:35:34PM +0800, Lai Jiangshan wrote:
> >>> I found that there is nothing to protect event_hash in
> >>> ftrace_find_event().
> >> Actually, rcu protects it, but not enough. We have neither
> >> synchronize_rcu() nor rcu_read_lock.
> >>
> >> So we protect against concurrent hlist accesses.
> >> But the event can be removed when a module is unloaded,
> >> and that can happen between the time we get the event output
> >> callback and the time we actually use it.
> > 
> > I will ask the stupid question...  Would invoking rcu_barrier() in the
> > module-exit function take care of this?  The rcu_barrier() primitive
> > waits for all in-flight RCU callbacks to complete execution.
> > 
> > 							Thanx, Paul
> > 
> 
> We have no call_rcu* in it.

OK, I will ask the next stupid question...  Is it possible to use the
same trick rcu_barrier() uses, but for your events?

							Thanx, Paul