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SubjectRe: lockdep splat in CPU hotplug
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@suse.cz>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

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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