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

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Message-ID: <4A124080.7010400@cn.fujitsu.com>
Date:	Tue, 19 May 2009 13:15:44 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	paulmck@...ux.vnet.ibm.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()

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.

Thanx, Lai

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