Messages in this thread | | | Date | Fri, 13 Sep 2013 07:33:15 +0200 | From | Thomas Hellstrom <> | Subject | Re: [BUG] completely bonkers use of set_need_resched + VM_FAULT_NOPAGE |
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On 09/12/2013 11:50 PM, Maarten Lankhorst wrote: > Op 12-09-13 18:44, Thomas Hellstrom schreef: >> 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 <peterz@infradead.org> 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. > CONFIG_PROVE_LOCKING=y > > and hard grab that reserve lock within the fault handler, done.. lockdep will spit it out for you :p > > ~Maarten
Given that all copy_to_user / copy_from_user paths are actually hit during testing, right?
/Thomas
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