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Date: Tue, 19 May 2009 10:05:21 +0800
From: Lai Jiangshan <laijs@cn.fujitsu.com>
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To: Frederic Weisbecker <fweisbec@gmail.com>
CC: Ingo Molnar <mingo@elte.hu>, Steven Rostedt <rostedt@goodmis.org>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] tracing: add trace_event_read_lock()
References: <4A114806.7090302@cn.fujitsu.com> <20090518135930.GC4704@nowhere>
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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.

We have no rcu_read_lock(), RCU can not protects it.

> 
> 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.
> 

[...]

> It could be more fine grained.

I think it's fine-grained enough, write-side(modules loading/unloading)
is happened rarely. trace_event_read_lock() will not sleep very likely.

Thoughts?

> We could have a per event rwsem, and also place the
> protected read section only in trace_print_entry() which is the only racy window.
> 

print_trace_line() is the only racy window.
So I just protect print_trace_line()(except __ftrace_dump())

I protect loops which call print_trace_line(), it
reduces invoke-times:

trace_event_read_lock();
while (...) {
	...
	print_trace_line();
	...
}
trace_event_read_unlock();

Thanks!
Lai

> But I'm not sure it's that worthy since event removal is a rare thing.
> 
> So I guess this patch is fine.
> 
> 
> 
>