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Date: Thu, 12 Sep 2013 18:22:10 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Daniel Vetter <daniel.vetter@...ll.ch> Cc: 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 05:58:49PM +0200, Daniel Vetter wrote: > On Thu, Sep 12, 2013 at 5:43 PM, Peter Zijlstra <peterz@...radead.org> wrote: > >> The one in ttm is just bonghits to shut up lockdep: ttm can recurse > >> into it's own pagefault handler and then deadlock, the trylock just > >> keeps lockdep quiet. We've had that bug arise in drm/i915 due to some > >> fun userspace did and now have testcases for them. The right solution > >> to fix this is to use copy_to|from_user_atomic in ttm everywhere it > >> holds locks and have slowpaths which drops locks, copies stuff into a > >> temp allocation and then continues. At least that's how we've fixed > >> all those inversions in i915-gem. I'm not volunteering to fix this ;-) > > > > Yikes.. so how common is it? If I simply rip the set_need_resched() out > > it will 'spin' on the fault a little longer until a 'natural' preemption > > point -- if such a thing is every going to happen. > > It's a case of "our userspace doesn't do this", so as long as you're > not evil and frob the drm device nodes of ttm drivers directly the > deadlock will never happen. No idea how much contention actually > happens on e.g. shared buffer objects - in i915 we have just one lock > and so suffer quite a bit more from contention. So no idea how much > removing the yield would hurt. If 'sane' userspace is never supposed to do this, then only insane userspace is going to hurt from this and that's a GOOD (tm) thing, right? ;-) And it won't actually deadlock if you don't use FIFO, for the regular scheduler class it'll just spin a little longer before getting preempted so no real worries there. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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