Date: Thu, 12 Sep 2013 22:30:20 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Chris Wilson <chris@chris-wilson.co.uk>,
        Daniel Vetter <daniel.vetter@ffwll.ch>, Dave Airlie <airlied@linux.ie>,
        Maarten Lankhorst <maarten.lankhorst@canonical.com>,
        Thomas Hellstrom <thellstrom@vmware.com>,
        intel-gfx <intel-gfx@lists.freedesktop.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [BUG] completely bonkers use of set_need_resched +
 VM_FAULT_NOPAGE
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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.
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