linux-kernel - Re: Question on lockdep and MAX_LOCK

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Date:	Tue, 05 Feb 2013 21:52:04 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ben Greear <greearb@...delatech.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: Question on lockdep and MAX_LOCK_DEPTH

On Tue, 2013-02-05 at 18:26 -0800, Ben Greear wrote:
> Well, here it is..something is calling rcu_read_lock lots and lots,

Or a bug in the way lockdep handles rcu mappings.

> it seems.  Any way to get a better idea of where those calls are
> made?

Yeah, with ftrace.

> 96 locks held by swapper/0/0:
>   #0:  (rcu_read_lock){.+.+..}, at: [<ffffffff81476836>] rcu_read_lock+0x0/0x6f
>   #1:  (rcu_read_lock){.+.+..}, at: [<ffffffff81476836>] rcu_read_lock+0x0/0x6f
[...]
>   #92:  (rcu_read_lock){.+.+..}, at: [<ffffffff81476836>] rcu_read_lock+0x0/0x6f
>   #93:  (&(&wl->cfg_spin_lock)->rlock){..-...}, at: [<ffffffffa07bc4d0>] handle_rcv+0x15d/0x1dd [wanlink]
>   #94:  (&wl_threads[q].my_wq){..-...}, at: [<ffffffff810abe4d>] __wake_up+0x1d/0x48
>   #95:  (&p->pi_lock){-.-.-.}, at: [<ffffffff810b09c1>] try_to_wake_up+0x29/0x20b

If you haven't already configured ftrace into your kernel, can you
please do so. Specifically:

CONFIG_FUNCTION_TRACER=y
CONFIG_FUNCTION_GRAPH_TRACER=y
CONFIG_DYNAMIC_FTRACE=y

Then, before triggering this, run the following as root:

 # mount -t debugfs nodev /sys/kernel/debug
 # cd /sys/kernel/debug/tracing
 # echo net_rx_action > set_graph_function
 # echo function_graph > current_tracer

In the kernel, where you added the above dump, before any of the printks
happen, add this too:

	trace_printk("BUG\n");
	tracing_off();

This will stop the trace at the point of the error. The trace_printk()
is a nice way to see the trace too.

Then after you trigger the bug, do the following:

  cat /sys/kernel/debug/tracing/trace

and reply with that.

To turn tracing on again (because the tracing_off() disabled it), you
can do:

  echo 1 > /sys/kernel/debug/tracing/tracing_on

Careful not to do "echo 1> ..." ;-)

-- Steve




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