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

[Mike Galbraith <umgwanakikbuti@gmail.com>]

On Thu, 2016-03-24 at 11:44 +0100, Thomas Gleixner wrote:

> 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.

It seems it's not the state machinery making a difference after all,
the only two deadlocks encountered in oodles of beating seem to boil
down to the grab_lock business being a pistol aimed at our own toes.

1. kernfs_mutex taken during hotplug: We don't pin across mutex
acquisition, so anyone grabbing it and then calling migrate_disable()
while grab_lock is set renders us dead.  Pin across acquisition of that
specific mutex fixes that specific grab_lock instigated deadlock.

2. notifier dependency upon RCU GP threads:  Telling same to always do
migrate_me() or hotplug can bloody well wait fixes that specific
grab_lock instigated deadlock.

With those two little hacks, all of my boxen including DL980 just keep
on chugging away in 4.[456]-rt, showing zero inclination to identify
any more hotplug bandits.

What I like much better than 1 + 2 is their sum, which would generate
minus signs, my favorite thing in patches, and fix the two above and
anything that resembles them in any way...

3. nuke irksome grab_lock: make everybody always try to get the hell
outta Dodge or hotplug can bloody well wait.

I haven't yet flogged my 64 core box doing that, but my local boxen
seem to be saying we don't really really need the grab_lock business.

Are my boxen fibbing, is that very attractive looking door #3 a trap?

	-Mike