linux-kernel - Re: [PATCH] tracing: add trace_event_read

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Date:	Tue, 19 May 2009 05:38:53 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing: add trace_event_read_lock()

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
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