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@xxxxxxxxxxxxx> 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 _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
