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

[Tvrtko Ursulin]

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?

Regards,

Tvrtko