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Message-ID: <4A1213E1.8050608@cn.fujitsu.com>
Date: Tue, 19 May 2009 10:05:21 +0800
From: Lai Jiangshan <laijs@...fujitsu.com>
To: Frederic Weisbecker <fweisbec@...il.com>
CC: 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()
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.
>
>
>
>
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