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Subject: Re: [PATCH] tracing: fix circular dead lock
From: Steven Rostedt <rostedt@goodmis.org>
To: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>, Frederic Weisbecker <fweisbec@gmail.com>,
       Ingo Molnar <mingo@elte.hu>, LKML <linux-kernel@vger.kernel.org>
In-Reply-To: <4B67D4F9.9080905@cn.fujitsu.com>
References: <4B67D4F9.9080905@cn.fujitsu.com>
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Date: Tue, 02 Feb 2010 08:25:35 -0500
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On Tue, 2010-02-02 at 15:32 +0800, Lai Jiangshan wrote:
> When we cat <debugfs>/tracing/stack_trace, we may cause circular lock:
> sys_read()
>   t_start()
>      arch_spin_lock(&max_stack_lock);
> 
>   t_show()
>      seq_printf(), vsnprintf() .... /* they are all trace-able,
>        when they are traced, max_stack_lock may be required again. */
> 
> 
> The following script can trigger this circular dead lock very easy:
> #!/bin/bash
> 
> echo 1 > /proc/sys/kernel/stack_tracer_enabled
> 
> mount -t debugfs xxx /mnt > /dev/null 2>&1

> (
> # make check_stack() zealous to require max_stack_lock
> for ((; ;))
> {
> 	echo 1 > /mnt/tracing/stack_max_size
> }
> ) &
> 
> for ((; ;))
> {
> 	cat /mnt/tracing/stack_trace > /dev/null
> }
> 
> 
> To fix this bug, we increase the percpu trace_active before
> require the lock.
> 
> Reported-by: Li Zefan <lizf@cn.fujitsu.com>
> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>

Thanks Lai! I'll try to get this into 33.

-- Steve