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 12:05:40 -0500
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Thomas Gleixner <tglx@...utronix.de>
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

* Thomas Gleixner (tglx@...utronix.de) wrote:
> 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.

Oh, right. Thanks for the explanation. I'll look into moving LTTng to a
saner del_timer_sync() scheme to delete the timers.

Thanks,

Mathieu

> 
> Thanks,
> 
> 	tglx
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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