[Intel-gfx] [BUG] completely bonkers use of set_need_resched + VM_FAULT_NOPAGE

[Peter Zijlstra]

On Thu, Sep 12, 2013 at 05:35:43PM +0100, Chris Wilson wrote:
> Not quite, as it would be possible for the evil userspace to trigger a
> GPU hang that would cause the sane userspace to spin indefinitely 
> waiting for the error recovery to kick in.

So with FIFOn+1 preempting FIFOn its a live-lock because the faulting
thread will forever keep yielding to itself since its the highest
priority task around, therefore the set_need_resched() is an absolute
NOP in that case.

For OTHER it might run another task with set_need_resched(), without
set_need_resched() it'll simply spin on the fault until it runs out of
time and gets force preempted and another task gets to run.

So for either case, the set_need_resched() doesn't make an appreciable
difference.

Removing it will not make evil userspace much worse -- at worst it will
cause slightly more wasted cycles.