[Intel-gfx] [PATCH] drm/i915: Use uninterruptible mutex_lock for userptr bo creation

[Chris Wilson]

On Mon, May 18, 2015 at 10:30:06AM +0200, Daniel Vetter wrote:
> On Fri, May 15, 2015 at 12:09:00PM +0100, Tvrtko Ursulin wrote:
> > 
> > Hi,
> > 
> > On 05/15/2015 11:42 AM, Chris Wilson wrote:
> > >Mika encountered one pathological scenario under X where acquiring all
> > >the mm locks (required to insert a mmu notifier) was very slow, so slow
> > >that by the time we tried to lock the struct_mutex with the usual call
> > >to i915_mutex_lock_interruptible(), X's signal timer had fired causing
> > >us to restart the ioctl (and so looped indefinitely).
> > 
> > Indefinite loop? Are you saying userptr creation endlessly fails to manages
> > to finish in 10ms (or is it even 100ms, forgot what timer Xorg setups up)?
> > The __mmu_notifier_register call?

Yes. In this scenario it is taking longer than 100ms to take all the mm locks.
I presume it is simply due to there being a vast number of mm on Mika's machine.

> > >While I suspect this is the result of another bug (something leaking mm
> > >perhaps?) we can forgo the error checking and interuptible nature of the
> > >lock here so we only have to pay the expense once and get on with it.
> > >This does expose the userptr creation routine to a driver livelock
> > >though by not being interruptible.
> > 
> > How is this acceptable then if it can live-lock? How does that happen?
> 
> If the i915 driver somehow dies it's a lot nicer for the user to be able
> to hit ^C and get out of trouble again and debug further than make
> anything touching i915 be stuck forever.
> 
> But I think this is a justified exception. Queued for -next, thanks for
> the patch.

Oops, gcc didn't warn about ret being used uninitialized. I guess I am
used to using it as a crutch.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre