On Thu, 24 Mar 2016, Mike Galbraith wrote: > 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@xxxxxxxxxxxxx> > > > > > > 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. Glad you found out yourself. Telling you that was on my todo list .... > 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. That's a given :) I really wonder what makes the change. The only thing which comes to my mind is the enforcement of running the online and down_prepare callbacks on the plugged cpu instead of doing it wherever the scheduler decides to run it. > 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. Did I miss a patch here or is that still in your pile? Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
