[BUG] completely bonkers use of set_need_resched + VM_FAULT_NOPAGE

Maarten Lankhorst maarten.lankhorst at canonical.com
Thu Sep 12 14:50:44 PDT 2013

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 at 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.

and hard grab that reserve lock within the fault handler, done.. lockdep will spit it out for you :p


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