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Date: Fri, 6 Mar 2009 12:08:00 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       LKML <linux-kernel@vger.kernel.org>,
       Steven Rostedt <srostedt@redhat.com>
Subject: Re: [PATCH for tip][bugfix] ftrace: fix deadlock when setting
	set_ftrace_pid
Message-ID: <20090306110800.GF21168@elte.hu>
References: <20090306151155.0778.A69D9226@jp.fujitsu.com> <49B0C94E.6020300@cn.fujitsu.com>
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* Lai Jiangshan <laijs@cn.fujitsu.com> wrote:

> KOSAKI Motohiro wrote:
> > Impact: fix deadlock bug
> > 
> > reproduce way:
> > 	# cd /sys/kernel/debug/tracing
> > 	# echo $$ > set_ftrace_pid
> > 
> > 	then, console become hangup.
> > 
> > detail:
> > when writing set_ftracepid, kernel callstack is following
> > 
> > 	ftrace_pid_write()
> > 		mutex_lock(&ftrace_lock);
> > 		ftrace_update_pid_func()
> > 			mutex_lock(&ftrace_lock);
> > 			mutex_unlock(&ftrace_lock);
> > 		mutex_unlock(&ftrace_lock);
> > 
> > then, system alaways deadlock when ftrace_pid_write() is called.
> > 
> > In past days, ftrace_pid_write() use ftrace_start_lock, but
> > commit e6ea44e9b4c12325337cd1c06103cd515a1c02b2 consolidate
> > ftrace_start_lock to ftrace_lock.
> > 
> > 
> > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> > ---
> > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > index 5a3a06b..0d3b393 100644
> > --- a/kernel/trace/ftrace.c
> > +++ b/kernel/trace/ftrace.c
> > @@ -218,10 +218,8 @@ static void ftrace_update_pid_func(void)
> >  {
> >  	ftrace_func_t func;
> >  
> > -	mutex_lock(&ftrace_lock);
> > -
> >  	if (ftrace_trace_function == ftrace_stub)
> > -		goto out;
> > +		return;
> >  
> >  	func = ftrace_trace_function;
> >  
> > @@ -238,9 +236,6 @@ static void ftrace_update_pid_func(void)
> >  #else
> >  	__ftrace_trace_function = func;
> >  #endif
> > -
> > - out:
> > -	mutex_unlock(&ftrace_lock);
> >  }
> >  
> >  /* set when tracing only a pid */
> > 
> > 
> 
> Reviewed-By: Lai Jiangshan <laijs@cn.fujitsu.com>

Applied, thanks!

	Ingo