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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 Wed, 2011-02-23 at 08:59 -0500, Mathieu Desnoyers wrote:
> > * 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
>
> 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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