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SubjectRe: [PATCH RFC tip/core/rcu 05/11] rcu: add comment saying why DEBUG_OBJECTS_RCU_HEAD depends on PREEMPT.
* Steven Rostedt (rostedt@goodmis.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@linux.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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