Re: [PATCH RT 4/6] rt/locking: Reenable migration accross schedule

[Mike Galbraith <umgwanakikbuti@gmail.com>]

On Sun, 2016-03-20 at 09:43 +0100, Mike Galbraith wrote:
> On Sat, 2016-02-13 at 00:02 +0100, Sebastian Andrzej Siewior wrote:
> > From: Thomas Gleixner <tglx@linutronix.de>
> > 
> > We currently disable migration across lock acquisition. That includes the part
> > where we block on the lock and schedule out. We cannot disable migration after
> > taking the lock as that would cause a possible lock inversion.
> > 
> > But we can be smart and enable migration when we block and schedule out. That
> > allows the scheduler to place the task freely at least if this is the first
> > migrate disable level. For nested locking this does not help at all.
> 
> I met a problem while testing shiny new hotplug machinery.
> 
> rt/locking: Fix rt_spin_lock_slowlock() vs hotplug migrate_disable() bug
> 
> migrate_disable() -> pin_current_cpu() -> hotplug_lock() leads to..
> 	> BUG_ON(rt_mutex_real_waiter(task->pi_blocked_on));
> ..so let's call migrate_disable() after we acquire the lock instead.

Well crap, that wasn't very clever  A little voice kept nagging me, and
yesterday I realized what it was grumbling about, namely that doing
migrate_disable() after lock acquisition will resurrect a hotplug
deadlock that we fixed up a while back.

On the bright side, with the busted migrate enable business reverted,
plus one dinky change from me [1], master-rt.today has completed 100
iterations of Steven's hotplug stress script along side endless
futexstress, and is happily doing another 900 as I write this, so the
next -rt should finally be hotplug deadlock free.

Thomas's state machinery seems to work wonders.  'course this being
hotplug, the other shoe will likely apply itself to my backside soon.

	-Mike

1. nest module_mutex inside hotplug_lock to prevent bloody systemd
-udevd from blocking in migrate_disable() while holding kernfs_mutex
during module load, putting a quick end to hotplug stress testing.