linux-kernel - Re: [ltt-dev] LTTng0.158 Linux-2629-RT kernel BUG: sleeping function called from invalid context at kernel/rtmutex.c:685

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Date:	Tue, 16 Feb 2010 18:01:49 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
cc:	Steven Rostedt <rostedt@...dmis.org>,
	ltt-dev@...ts.casi.polymtl.ca, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org
Subject: Re: [ltt-dev] LTTng0.158 Linux-2629-RT kernel BUG: sleeping function
 called from invalid context at kernel/rtmutex.c:685

On Tue, 16 Feb 2010, Mathieu Desnoyers wrote:
> > The function is called from an IPI. That's a LTTNG problem, not a RT one.
> 
> I use del_timer in IPI to delete lttng per-cpu timers on all CPUs. I
> have to do this because timers created with add_timer_on are documented
> to be incompatible with del_timer_sync():
> 
>  * Synchronization rules: Callers must prevent restarting of the timer,
>  * otherwise this function is meaningless. It must not be called from
>  * interrupt contexts. The caller must not hold locks which would prevent
>  * completion of the timer's handler. The timer's handler must not call
>  * add_timer_on(). Upon exit the timer is not queued and the handler is
>  * not running on any CPU.

Errm. The documentation says: 

      "The timer's handler must not call add_timer_on()." 

It's not talking about a timer which was initialized with
add_timer_on().

 And your per cpu timer handlers have no requirement to call
add_timer_on() simply because add/mod_timer() is requeueing the timer
on the same cpu on which the handler runs. 

So the IPI is just a solution for a non existing problem.

Thanks,

	tglx
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