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SubjectRe: [PATCH 1/4] rcu: Don't check irq nesting from rcu idle entry/exit
On Tue, Nov 29, 2011 at 01:26:01AM +0100, Frederic Weisbecker wrote:
> On Mon, Nov 28, 2011 at 04:22:44PM -0800, Paul E. McKenney wrote:
> > On Mon, Nov 28, 2011 at 10:24:44PM +0100, Frederic Weisbecker wrote:
> > > rcu_idle_enter() and rcu_idle_exit() don't need to check
> > > the irq nesting as they are not called from irq.
> > >
> > > Only check dyntick_nesting from rcu_irq_enter() and
> > > rcu_irq_exit().
> > >
> > > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> > > Cc: Josh Triplett <josh@joshtriplett.org>
> > > ---
> > > kernel/rcutree.c | 18 ++++++++----------
> > > 1 files changed, 8 insertions(+), 10 deletions(-)
> >
> > I have queued this. I had to update it to deal with a recent conflicting
> > update, please see below for the new version.
> >
> > Thanx, Paul
>
> Hmm, seems there is a problem. Look below:

Good eyes! How about the following instead?

------------------------------------------------------------------------

rcu: Don't check irq nesting from rcu idle entry/exit

Because tasks do not nest, rcu_idle_enter() and rcu_idle_exit() do
not need to check for nesting. This commit therefore moves nesting
checks from rcu_idle_enter_common() to rcu_irq_exit() and from
rcu_idle_exit_common() to rcu_irq_enter().

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index bf085d7..860c02c 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -350,10 +350,6 @@ static int rcu_implicit_offline_qs(struct rcu_data *rdp)
*/
static void rcu_idle_enter_common(struct rcu_dynticks *rdtp, long long oldval)
{
- if (rdtp->dynticks_nesting) {
- trace_rcu_dyntick("--=", oldval, rdtp->dynticks_nesting);
- return;
- }
trace_rcu_dyntick("Start", oldval, rdtp->dynticks_nesting);
if (!is_idle_task(current)) {
struct task_struct *idle = idle_task(smp_processor_id());
@@ -426,7 +422,10 @@ void rcu_irq_exit(void)
oldval = rdtp->dynticks_nesting;
rdtp->dynticks_nesting--;
WARN_ON_ONCE(rdtp->dynticks_nesting < 0);
- rcu_idle_enter_common(rdtp, oldval);
+ if (rdtp->dynticks_nesting)
+ trace_rcu_dyntick("--=", oldval, rdtp->dynticks_nesting);
+ else
+ rcu_idle_enter_common(rdtp, oldval);
local_irq_restore(flags);
}

@@ -439,10 +438,6 @@ void rcu_irq_exit(void)
*/
static void rcu_idle_exit_common(struct rcu_dynticks *rdtp, long long oldval)
{
- if (oldval) {
- trace_rcu_dyntick("++=", oldval, rdtp->dynticks_nesting);
- return;
- }
smp_mb__before_atomic_inc(); /* Force ordering w/previous sojourn. */
atomic_inc(&rdtp->dynticks);
/* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
@@ -518,7 +513,10 @@ void rcu_irq_enter(void)
oldval = rdtp->dynticks_nesting;
rdtp->dynticks_nesting++;
WARN_ON_ONCE(rdtp->dynticks_nesting == 0);
- rcu_idle_exit_common(rdtp, oldval);
+ if (oldval)
+ trace_rcu_dyntick("++=", oldval, rdtp->dynticks_nesting);
+ else
+ rcu_idle_exit_common(rdtp, oldval);
local_irq_restore(flags);
}



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