Re: [PATCH] drm/i915: Preallocate mmu notifier to unbreak cpu hotplug deadlock

[Thomas Gleixner]

On Thu, 5 Oct 2017, Daniel Vetter wrote:
> On Thu, Oct 05, 2017 at 06:19:30PM +0200, Thomas Gleixner wrote:
> > On Thu, 5 Oct 2017, Daniel Vetter wrote:
> > > On Thu, Oct 05, 2017 at 05:23:20PM +0200, Thomas Gleixner wrote:
> > > > Aside of that, is it really required to use stomp_machine() for this
> > > > synchronization? We certainly have less intrusive mechansisms than that.
> > > 
> > > Yeah, the stop_machine needs to go, I'm working on something that uses
> > > rcu_read_lock+synchronize_rcu for this case. Probably shouldn't have
> > > merged even.
> > > 
> > > Now this one isn't the one I wanted to fix with this patch since there's
> > > clearly something dubious going on on the i915 side too.
> > 
> > I already wondered :)
> > 
> > > The proper trace, with the same part on the cpu hotplug side, highlights
> > > that you can't allocate a workqueue while hodling mmap_sem. That one
> > > matches patch description&diff a bit better :-)
> > 
> > > Sorry for misleading you, should have checked to attach the right one. No
> > > stop_machine()/i915_gem_set_wedged() in the below one.
> > 
> > Well the problem is more or less the same and what I said about solving it
> > in a different place is still valid. I think about it some more, but don't
> > expect wonders :)
> 
> Yeah just want to make you aware there's now new implications in the
> locking maze and that we overall decide to break the loop in the right
> place. Also adding Tejun, since this is about workqueues, I forgot him.
> 
> tldr for Tejun: The new cross-release stuff in lockdep seems to indicate
> that we cannot allocate a new workqueue while holding mmap_sem. Full
> details in the thread.

The issue is not restricted to work queues and mmap_sem. There is the
general problem of:

 cpuhotplug -> cpu_up/down -> callback -> device_create/destroy()

which creates a dependency between

      cpuhotplug_rwsem and devfs locking

So now any chain which either holds a devfs lock or has a separate
dependecy chain on the devfs locks and then calls some function which tries
to take cpuhotplug_rwsem will trigger a splat. Rightfully so ....

So in your case mmap_sem is involved in that, but that's not a
prerequisite. There are a gazillion other ways to achieve that.

The pattern which causes that is device creation in a hotplug callback and
then some other device access (read/write/ioctl) which ends up to acquire
cpuhotplug_rwsem plus a connection of both chains through arbitrary locks.

I'm trying to move out that decice_create/remove stuff from the regular
hotplug states, but I haven't found a solution for that yet which is
neither butt ugly nor creates other hard to solve problems.

Maybe a glas of wine or some sleep or both will help to get over that :)

Surely anyone is welcome to beat me to it.

Thanks,

	tglx