linux-kernel - Re: [BUG] completely bonkers use of set_need_resched + VM_FAULT

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Message-ID: <20130912203020.GB1798@laptop.programming.kicks-ass.net>
Date:	Thu, 12 Sep 2013 22:30:20 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Chris Wilson <chris@...is-wilson.co.uk>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Dave Airlie <airlied@...ux.ie>,
	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	Thomas Hellstrom <thellstrom@...are.com>,
	intel-gfx <intel-gfx@...ts.freedesktop.org>,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [BUG] completely bonkers use of set_need_resched +
 VM_FAULT_NOPAGE

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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