linux-kernel - Re: [PATCH tip/core/urgent 1/7] rcu: decrease rcu_report_exp

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Message-ID: <20110720045414.GC2400@linux.vnet.ibm.com>
Date:	Tue, 19 Jul 2011 21:54:15 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, patches@...aro.org,
	greearb@...delatech.com, edt@....ca
Subject: Re: [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@....ca>
Signed-off-by: Paul E. McKenney <paulmck@...ux.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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