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SubjectRe: [PATCH tip/core/urgent 1/7] rcu: decrease rcu_report_exp_rnp coupling with scheduler
On Wed, Jul 20, 2011 at 04:40:18AM +0200, Peter Zijlstra wrote:
> On Tue, 2011-07-19 at 17:18 -0700, Paul E. McKenney wrote:
> > +++ b/kernel/rcutree_plugin.h
> > @@ -696,8 +696,10 @@ static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp)
> > raw_spin_lock_irqsave(&rnp->lock, flags);
> > for (;;) {
> > if (!sync_rcu_preempt_exp_done(rnp))
> > + raw_spin_unlock_irqrestore(&rnp->lock, flags);
> > break;
>
> I bet that'll all work much better if you wrap it in curly braces like:
>
> if (!sync_rcu_preempt_exp_done(rnp)) {
> raw_spin_unlock_irqrestore(&rnp->lock, flags);
> break;
> }
>
> That might also explain those explosions Ed and Ben have been seeing.

Indeed. Must be the call of the snake. :-(

Thank you for catching this!

> > if (rnp->parent == NULL) {
> > + raw_spin_unlock_irqrestore(&rnp->lock, flags);
> > wake_up(&sync_rcu_preempt_exp_wq);
> > break;
> > }
> > @@ -707,7 +709,6 @@ static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp)
> > raw_spin_lock(&rnp->lock); /* irqs already disabled */
> > rnp->expmask &= ~mask;
> > }
> > - raw_spin_unlock_irqrestore(&rnp->lock, flags);
> > }

So this time I am testing the exact patch series before resending.
In the meantime, here is the updated version of this patch.

Thanx, Paul

------------------------------------------------------------------------
rcu: decrease rcu_report_exp_rnp coupling with scheduler

PREEMPT_RCU read-side critical sections blocking an expedited grace
period invoke rcu_report_exp_rnp(). When the last such critical section
has completed, rcu_report_exp_rnp() invokes the scheduler to wake up the
task that invoked synchronize_rcu_expedited() -- needlessly holding the
root rcu_node structure's lock while doing so, thus needlessly providing
a way for RCU and the scheduler to deadlock.

This commit therefore releases the root rcu_node structure's lock before
calling wake_up().

Reported-by: Ed Tomlinson <edt@aei.ca>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 75113cb..6abef3c 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -695,9 +695,12 @@ static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp)

raw_spin_lock_irqsave(&rnp->lock, flags);
for (;;) {
- if (!sync_rcu_preempt_exp_done(rnp))
+ if (!sync_rcu_preempt_exp_done(rnp)) {
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
break;
+ }
if (rnp->parent == NULL) {
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
wake_up(&sync_rcu_preempt_exp_wq);
break;
}
@@ -707,7 +710,6 @@ static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp)
raw_spin_lock(&rnp->lock); /* irqs already disabled */
rnp->expmask &= ~mask;
}
- raw_spin_unlock_irqrestore(&rnp->lock, flags);
}

/*

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