linux-kernel - Re: [PATCH tip/core/rcu 09/15] rcu: Avoid rcu_print_detail_task_stall

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Date:	Tue, 4 Sep 2012 15:55:42 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org
Subject: Re: [PATCH tip/core/rcu 09/15] rcu: Avoid
 rcu_print_detail_task_stall_rnp() segfault

On Tue, Sep 04, 2012 at 03:46:59PM -0700, Paul E. McKenney wrote:
> On Fri, Aug 31, 2012 at 11:19:17AM -0700, Josh Triplett wrote:
> > On Thu, Aug 30, 2012 at 11:56:22AM -0700, Paul E. McKenney wrote:
> > > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > > 
> > > The rcu_print_detail_task_stall_rnp() function invokes
> > > rcu_preempt_blocked_readers_cgp() to verify that there are some preempted
> > > RCU readers blocking the current grace period outside of the protection
> > > of the rcu_node structure's ->lock.  This means that the last blocked
> > > reader might exit its RCU read-side critical section and remove itself
> > > from the ->blkd_tasks list before the ->lock is acquired, resulting in
> > > a segmentation fault when the subsequent code attempts to dereference
> > > the now-NULL gp_tasks pointer.
> > > 
> > > This commit therefore moves the test under the lock.  This will not
> > > have measurable effect on lock contention because this code is invoked
> > > only when printing RCU CPU stall warnings, in other words, in the common
> > > case, never.
> > > 
> > > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > ---
> > >  kernel/rcutree_plugin.h |    6 ++++--
> > >  1 files changed, 4 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> > > index 139a803..c02dc1d 100644
> > > --- a/kernel/rcutree_plugin.h
> > > +++ b/kernel/rcutree_plugin.h
> > > @@ -422,9 +422,11 @@ static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
> > >  	unsigned long flags;
> > >  	struct task_struct *t;
> > >  
> > > -	if (!rcu_preempt_blocked_readers_cgp(rnp))
> > > -		return;
> > >  	raw_spin_lock_irqsave(&rnp->lock, flags);
> > > +	if (!rcu_preempt_blocked_readers_cgp(rnp)) {
> > > +		raw_spin_unlock_irqrestore(&rnp->lock, flags);
> > > +		return;
> > > +	}
> > >  	t = list_entry(rnp->gp_tasks,
> > >  		       struct task_struct, rcu_node_entry);
> > >  	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry)
> > 
> > Given the small number of lines of code inside the critical section
> > here, I think this would look clearer without the early return and
> > duplicate lock release:
> > 
> > 	raw_spin_lock_irqsave(&rnp->lock, flags);
> > 	if (rcu_preempt_blocked_readers_cgp(rnp)) {
> > 		...
> > 	}
> > 	raw_spin_unlock_irqrestore(&rnp->lock, flags);
> 
> You might well be right, but doing that gets me another line longer
> than 80 characters.

Even with that line broken in an appropriate place, the result still
seems clearer.

> Hey, I have an excuse -- I actually spent a significant fraction of
> my career using punched cards.  ;-)

:)

- Josh Triplett
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