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

[Daniel Vetter]

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?
> 
> >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.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch