Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754767Ab3ILQoS (ORCPT ); Thu, 12 Sep 2013 12:44:18 -0400 Received: from smtp-outbound-1.vmware.com ([208.91.2.12]:56096 "EHLO smtp-outbound-1.vmware.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753871Ab3ILQoR (ORCPT ); Thu, 12 Sep 2013 12:44:17 -0400 Message-ID: <5231EF5A.7010901@vmware.com> Date: Thu, 12 Sep 2013 18:44:10 +0200 From: Thomas Hellstrom User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130625 Thunderbird/17.0.7 MIME-Version: 1.0 To: Maarten Lankhorst CC: Daniel Vetter , Peter Zijlstra , Dave Airlie , intel-gfx , dri-devel , Linux Kernel Mailing List , Ingo Molnar , Thomas Gleixner Subject: Re: [BUG] completely bonkers use of set_need_resched + VM_FAULT_NOPAGE References: <20130912150645.GZ31370@twins.programming.kicks-ass.net> <5231E18D.7070306@canonical.com> In-Reply-To: <5231E18D.7070306@canonical.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3306 Lines: 66 On 09/12/2013 05:45 PM, Maarten Lankhorst wrote: > Op 12-09-13 17:36, Daniel Vetter schreef: >> On Thu, Sep 12, 2013 at 5:06 PM, Peter Zijlstra wrote: >>> So I'm poking around the preemption code and stumbled upon: >>> >>> drivers/gpu/drm/i915/i915_gem.c: set_need_resched(); >>> drivers/gpu/drm/ttm/ttm_bo_vm.c: set_need_resched(); >>> drivers/gpu/drm/ttm/ttm_bo_vm.c: set_need_resched(); >>> drivers/gpu/drm/udl/udl_gem.c: set_need_resched(); >>> >>> All these sites basically do: >>> >>> while (!trylock()) >>> yield(); >>> >>> which is a horrible and broken locking pattern. >>> >>> Firstly its deadlock prone, suppose the faulting process is a FIFOn+1 >>> task that preempted the lock holder at FIFOn. >>> >>> Secondly the implementation is worse than usual by abusing >>> VM_FAULT_NOPAGE, which is supposed to install a PTE so that the fault >>> doesn't retry, but you're using it as a get out of fault path. And >>> you're using set_need_resched() which is not something a driver should >>> _ever_ touch. >>> >>> Now I'm going to take away set_need_resched() -- and while you can >>> 'reimplement' it using set_thread_flag() you're not going to do that >>> because it will be broken due to changes to the preempt code. >>> >>> So please as to fix ASAP and don't allow anybody to trick you into >>> merging silly things like that again ;-) >> The set_need_resched in i915_gem.c:i915_gem_fault can actually be >> removed. It was there to give the error handler a chance to sneak in >> and reset the hw/sw tracking when the gpu is dead. That hack goes back >> to the days when the locking around our error handler was somewhere >> between nonexistent and totally broken, nowadays we keep things from >> live-locking by a bit of magic in i915_mutex_lock_interruptible. I'll >> whip up a patch to rip this out. I'll also check that our testsuite >> properly exercises this path (needs a bit of work on a quick look for >> better coverage). >> >> 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 ;-) > Ah the case where a mmap'd address is passed to the execbuf ioctl? :P > > Fine I'll look into it a bit, hopefully before tuesday. Else it might take a bit longer since I'll be on my way to plumbers.. I think a possible fix would be if fault() were allowed to return an error and drop the mmap_sem() before returning. Otherwise we need to track down all copy_to_user / copy_from_user which happen with bo::reserve held. /Thomas -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/