From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [BUG] completely bonkers use of set_need_resched + VM_FAULT_NOPAGE Date: Thu, 12 Sep 2013 18:22:10 +0200 Message-ID: <20130912162210.GE31370@twins.programming.kicks-ass.net> References: <20130912150645.GZ31370@twins.programming.kicks-ass.net> <20130912154329.GB31370@twins.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Daniel Vetter Cc: Dave Airlie , Maarten Lankhorst , Thomas Hellstrom , intel-gfx , dri-devel , Linux Kernel Mailing List , Ingo Molnar , Thomas Gleixner List-Id: intel-gfx@lists.freedesktop.org On Thu, Sep 12, 2013 at 05:58:49PM +0200, Daniel Vetter wrote: > On Thu, Sep 12, 2013 at 5:43 PM, Peter Zijlstra 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.