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 09:37:48 -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 Wed, 2011-02-23 at 08:59 -0500, Mathieu Desnoyers wrote:
> > * 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
> 
> Looks like it was in rcuhead_fixup_free to me.

OK.

> 
> > 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 ?
> 
> The point is that this entire block of code is wrapped in #ifdef
> CONFIG_DEBUG_OBJECTS_RCU_HEAD, and that config depends on PREEMPT. Thus
> you will never have a case where #ifdef CONFIG_PREEMPT is false.

Why does CONFIG_DEBUG_OBJECTS_RCU_HEAD depend on PREEMPT again ?

Mathieu

> 
> -- Steve
> 
> 

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