Date:	Thu, 18 Feb 2010 09:28:00 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Ingo Molnar <mingo@...e.hu>, ltt-dev@...ts.casi.polymtl.ca,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Neil Horman <nhorman@...driver.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: add_timer_on: in-kernel users _all_ buggy ?

* Mathieu Desnoyers (compudj@...stal.dyndns.org) wrote:
> * 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.

Double-checking this:

add_timer_on() needs to be paired with mod_timer_pinned(), otherwise
NO_HZ SMP config can rebalance the timer to a different CPU. I am fixing
this in lttng 0.194. These per-cpu timers, of course, should usually be
deferrable (they are in lttng).

(looking at kernel 2.6.32.4 here)
Looking at the kernel/time/clocksource.c watchdog, I wonder how
del_timer manages to synchronize the timer teardown. The handler,
clocksource_watchdog(), uses add_timer_on(), which prohibits using
del_timer_sync(). This seems rather odd. If we remove the watchdog and
re-add it, it may still be in use while we initialize the timer
structure.

Also, net/core/drop_monitor.c trace_drop_common usage of add_timer_on
seems odd:

Executing (AFAIK) with preempt on, data points to a per-cpu timer:

       if (!timer_pending(&data->send_timer)) {
                data->send_timer.expires = jiffies + dm_delay * HZ;
                add_timer_on(&data->send_timer, smp_processor_id());
       }

How is timer_pending synchronized with the target CPU timer wheel ?

Wait, there's more: arch/x86/kernel/cpu/mcheck/mce.c uses both
add_timer_on in its handler and del_timer_sync (which is incorrect).

arch/x86/kernel/apic/x2apic_uv_x.c almost has it right, but maybe it
should use del_timer_sync ?

arch/powerpc/platforms/chrp/setup.c should learn about
mod_timer_pinned().

Which leads to the following question: is there _any_ add_timer_on()
kernel user that's not currently buggy ? ;-) Maybe this calls for better
documentation of this interface. From what I've learn from digging into
cpufreq to debug its incorrect timer teardown last year, I fear there
are lots and lots of buggy per-cpu _and_ standard timer interface users
out there.

Maybe adding some debugging options, e.g. checking that a timer created
with add_timer_on is always modified by mod_timer_pinned, and is always
deferrable, and deleted by del_timer_sync could help discovering a
couple of outlawyer.

Thanks,

Mathieu


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

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