linux-kernel - Re: [PATCH RFC tip/core/rcu 05/11] rcu: add comment saying why DEBUG_OBJECTS_RCU

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Date:	Wed, 23 Feb 2011 08:59:01 -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 RFC tip/core/rcu 05/11] rcu: add comment saying why
	DEBUG_OBJECTS_RCU_HEAD depends on PREEMPT.

* Steven Rostedt (rostedt@...dmis.org) wrote:
> On Tue, 2011-02-22 at 17:39 -0800, Paul E. McKenney wrote:
> > The build will break if you change the Kconfig to allow
> > DEBUG_OBJECTS_RCU_HEAD and !PREEMPT, so document the reasoning
> > near where the breakage would occur.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > ---
> >  kernel/rcupdate.c |    5 +++++
> >  1 files changed, 5 insertions(+), 0 deletions(-)
> > 
> > diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
> > index afd21d1..f3240e9 100644
> > --- a/kernel/rcupdate.c
> > +++ b/kernel/rcupdate.c
> > @@ -214,6 +214,11 @@ static int rcuhead_fixup_free(void *addr, enum debug_obj_state state)
> >  		 * 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.
> > +		 * Note that the machinery to reliably determine whether
> > +		 * or not we are in an RCU read-side critical section
> > +		 * exists only in the preemptible RCU implementations
> > +		 * (TINY_PREEMPT_RCU and TREE_PREEMPT_RCU), which is why
> > +		 * DEBUG_OBJECTS_RCU_HEAD is disallowed if !PREEMPT.
> >  		 */
> 
> Shouldn't this comment also be in the kconfig where
> DEBUG_OBJECTS_RCU_HEAD is defined?

hrm, but this is a "rcuhead_fixup_init" : it does not need to always
succeed. It's just that when it is safe to recover from an error
situation, it does it. We could do:

#ifdef CONFIG_PREEMPT
                /*
                 * 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.
                 */
                if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
                    irqs_disabled()) {
                        WARN_ON(1);
                        return 0;
                }
                rcu_barrier();
                rcu_barrier_sched();
                rcu_barrier_bh();
                debug_object_init(head, &rcuhead_debug_descr);
                return 1;
#else
		return 0;
#endif

instead, no ?

Mathieu

> 
> -- Steve
> 
> >  		if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
> >  		    irqs_disabled()) {
> 
> 

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