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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Feb 23, 2011 at 10:37:26AM -0500, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@goodmis.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@efficios.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.

Well, it would add noise, so I added a patch converting the WARN_ON's
to WARN_ON_ONCE's.

							Thanx, Paul