linux-kernel - Lockup in tracepoint unregister in sched switch ftrace plugin

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Date:	Mon, 20 Oct 2008 23:48:48 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Lockup in tracepoint unregister in sched switch ftrace plugin


Mathieu,

I just downloaded the latest git repo from Linus's tree, and the 
sched_switch start up test locks up. I traced it down to the first 
unregister of a trace point. Here's the call path that I see.

kernel/trace/trace.c: register_tracer
kernel/trace/trace_selftest.c: trace_selftest_startup_sched_switch
kernel/trace/trace_sched_switch.c: sched_switch_trace_ctrl_update
   " "  : stop_sched_trace
   " "  : tracing_stop_cmdline_record
   " "  : tracing_stop_sched_switch
   " "  : tracing_sched_unregister

  which calls unregister_trace_sched_switch define as macro to:

kernel/tracepoint.c: tracepoint_probe_unregister
   " "  : remove_tracepoint
kernel/rcupdate.c: rcu_barrier_sched
   " "  : _rcu_barrier

where it gets stuck at that "wait_for_completion".

I'm not sure if, because this is a scheduler trace point that we are 
hitting some kind of race that is preventing the wait_for_completion to 
finish, or what.

I'll look more at it tomorrow.

-- Steve


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