Re: lockdep splat in CPU hotplug

[Paul E. McKenney]

On Wed, Oct 22, 2014 at 09:54:33AM -0700, Paul E. McKenney wrote:
> On Wed, Oct 22, 2014 at 07:38:37AM -0700, Paul E. McKenney wrote:
> > On Wed, Oct 22, 2014 at 11:53:49AM +0200, Jiri Kosina wrote:
> > > On Tue, 21 Oct 2014, Jiri Kosina wrote:
> > > 
> > > > Hi,
> > > > 
> > > > I am seeing the lockdep report below when resuming from suspend-to-disk 
> > > > with current Linus' tree (c2661b80609).
> > > > 
> > > > The reason for CCing Ingo and Peter is that I can't make any sense of one 
> > > > of the stacktraces lockdep is providing.
> > > > 
> > > > Please have a look at the very first stacktrace in the dump, where lockdep 
> > > > is trying to explain where cpu_hotplug.lock#2 has been acquired. It seems 
> > > > to imply that cpuidle_pause() is taking cpu_hotplug.lock, but that's not 
> > > > the case at all.
> > > > 
> > > > What am I missing?
> > > 
> > > Okay, reverting 442bf3aaf55a ("sched: Let the scheduler see CPU idle 
> > > states") and followup 83a0a96a5f26 ("sched/fair: Leverage the idle state 
> > > info when choosing the "idlest" cpu") which depends on it makes the splat 
> > > go away.
> > > 
> > > Just for the sake of testing the hypothesis, I did just the minimal change 
> > > below on top of current Linus' tree, and it also makes the splat go away 
> > > (of course it's totally incorrect thing to do by itself alone):
> > > 
> > > diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> > > index 125150d..d31e04c 100644
> > > --- a/drivers/cpuidle/cpuidle.c
> > > +++ b/drivers/cpuidle/cpuidle.c
> > > @@ -225,12 +225,6 @@ void cpuidle_uninstall_idle_handler(void)
> > >  		initialized = 0;
> > >  		wake_up_all_idle_cpus();
> > >  	}
> > > -
> > > -	/*
> > > -	 * Make sure external observers (such as the scheduler)
> > > -	 * are done looking at pointed idle states.
> > > -	 */
> > > -	synchronize_rcu();
> > >  }
> > > 
> > >  /**
> > > 
> > > So indeed 442bf3aaf55a is guilty.
> > > 
> > > Paul was stating yesterday that it can't be the try_get_online_cpus() in 
> > > synchronize_sched_expedited(), as it's doing only trylock. There are 
> > > however more places where synchronize_sched_expedited() is acquiring 
> > > cpu_hotplug.lock unconditionally by calling put_online_cpus(), so the race 
> > > seems real.
> > 
> > Gah!  So I only half-eliminated the deadlock between
> > synchronize_sched_expedited() and CPU hotplug.  Back to the drawing
> > board...
> 
> Please see below for an untested alleged fix.

And that patch had a lockdep issue.  The following replacement patch
passes light rcutorture testing, but your mileage may vary.

							Thanx, Paul

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

rcu: More on deadlock between CPU hotplug and expedited grace periods

Commit dd56af42bd82 (rcu: Eliminate deadlock between CPU hotplug and
expedited grace periods) was incomplete.  Although it did eliminate
deadlocks involving synchronize_sched_expedited()'s acquisition of
cpu_hotplug.lock via get_online_cpus(), it did nothing about the similar
deadlock involving acquisition of this same lock via put_online_cpus().
This deadlock became apparent with testing involving hibernation.

This commit therefore changes put_online_cpus() acquisition of this lock
to be conditional, and increments a new cpu_hotplug.puts_pending field
in case of acquisition failure.  Then cpu_hotplug_begin() checks for this
new field being non-zero, and applies any changes to cpu_hotplug.refcount.

Reported by: Jiri Kosina <jkosina@xxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 356450f09c1f..90a3d017b90c 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -64,6 +64,8 @@ static struct {
 	 * an ongoing cpu hotplug operation.
 	 */
 	int refcount;
+	/* And allows lockless put_online_cpus(). */
+	atomic_t puts_pending;
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map dep_map;
@@ -113,7 +115,11 @@ void put_online_cpus(void)
 {
 	if (cpu_hotplug.active_writer == current)
 		return;
-	mutex_lock(&cpu_hotplug.lock);
+	if (!mutex_trylock(&cpu_hotplug.lock)) {
+		atomic_inc(&cpu_hotplug.puts_pending);
+		cpuhp_lock_release();
+		return;
+	}
 
 	if (WARN_ON(!cpu_hotplug.refcount))
 		cpu_hotplug.refcount++; /* try to fix things up */
@@ -155,6 +161,12 @@ void cpu_hotplug_begin(void)
 	cpuhp_lock_acquire();
 	for (;;) {
 		mutex_lock(&cpu_hotplug.lock);
+		if (atomic_read(&cpu_hotplug.puts_pending)) {
+			int delta;
+
+			delta = atomic_xchg(&cpu_hotplug.puts_pending, 0);
+			cpu_hotplug.refcount -= delta;
+		}
 		if (likely(!cpu_hotplug.refcount))
 			break;
 		__set_current_state(TASK_UNINTERRUPTIBLE);

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