linux-kernel - Re: [PATCH] debug rcu head support !PREEMPT config

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Message-ID: <BLU0-SMTP85C4E764892C36C997713096DB0@phx.gbl>
Date:	Wed, 23 Feb 2011 10:37:26 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com
Subject: Re: [PATCH] debug rcu head support !PREEMPT config

* Steven Rostedt (rostedt@...dmis.org) wrote:
> On Wed, 2011-02-23 at 10:13 -0500, Mathieu Desnoyers wrote:
> > Remove DEBUG_RCU_HEAD dependency on PREEMPT config. Handle the unability to
> > detect if within a RCU read-side critical section by never performing any
> > attempt to recover from a failure situation in the fixup handlers. Just print
> > the warnings.
> > 
> > This patch is only compile-tested.
> > 
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > ---
> >  kernel/rcupdate.c |   17 +++++++++++++++++
> >  lib/Kconfig.debug |    2 +-
> >  2 files changed, 18 insertions(+), 1 deletion(-)
> > 
> > Index: linux-2.6-lttng/lib/Kconfig.debug
> > ===================================================================
> > --- linux-2.6-lttng.orig/lib/Kconfig.debug
> > +++ linux-2.6-lttng/lib/Kconfig.debug
> > @@ -313,7 +313,7 @@ config DEBUG_OBJECTS_WORK
> >  
> >  config DEBUG_OBJECTS_RCU_HEAD
> >  	bool "Debug RCU callbacks objects"
> > -	depends on DEBUG_OBJECTS && PREEMPT
> > +	depends on DEBUG_OBJECTS
> >  	help
> >  	  Enable this to turn on debugging of RCU list heads (call_rcu() usage).
> >  
> > Index: linux-2.6-lttng/kernel/rcupdate.c
> > ===================================================================
> > --- linux-2.6-lttng.orig/kernel/rcupdate.c
> > +++ linux-2.6-lttng/kernel/rcupdate.c
> > @@ -142,7 +142,14 @@ static int rcuhead_fixup_init(void *addr
> >  		 * Ensure that queued callbacks are all executed.
> >  		 * If we detect that we are nested in a RCU read-side critical
> >  		 * section, we should simply fail, otherwise we would deadlock.
> > +		 * In !PREEMPT configurations, there is no way to tell if we are
> > +		 * in a RCU read-side critical section or not, so we never
> > +		 * attempt any fixup and just print a warning.
> >  		 */
> > +#ifndef CONFIG_PREEMPT
> > +		WARN_ON(1);
> > +		return 0;
> > +#endif
> >  		if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
> >  		    irqs_disabled()) {
> >  			WARN_ON(1);
> > @@ -184,7 +191,14 @@ static int rcuhead_fixup_activate(void *
> >  		 * Ensure that queued callbacks are all executed.
> >  		 * If we detect that we are nested in a RCU read-side critical
> >  		 * section, we should simply fail, otherwise we would deadlock.
> > +		 * In !PREEMPT configurations, there is no way to tell if we are
> > +		 * in a RCU read-side critical section or not, so we never
> > +		 * attempt any fixup and just print a warning.
> >  		 */
> > +#ifndef CONFIG_PREEMPT
> > +		WARN_ON(1);
> > +		return 0;
> > +#endif
> >  		if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
> >  		    irqs_disabled()) {
> >  			WARN_ON(1);
> > @@ -214,6 +228,9 @@ static int rcuhead_fixup_free(void *addr
> >  		 * Ensure that queued callbacks are all executed.
> >  		 * If we detect that we are nested in a RCU read-side critical
> >  		 * section, we should simply fail, otherwise we would deadlock.
> > +		 * In !PREEMPT configurations, there is no way to tell if we are
> > +		 * in a RCU read-side critical section or not, so we never
> > +		 * attempt any fixup and just print a warning.
> >  		 */
> >  #ifndef CONFIG_PREEMPT
> >  		WARN_ON(1);
> 
> Hmm, I wonder if s/WARN_ON/WARN_ON_ONCE/g is in order. Why spam the
> console if it happens to trigger all the time?

The system should die pretty soon anyway due to list corruption, so I
don't think it's a problem in practice.

Thanks,

Mathieu

> 
> -- Steve
> 
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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