[BUG] completely bonkers use of set_need_resched + VM_FAULT_NOPAGE

Maarten Lankhorst maarten.lankhorst at canonical.com
Fri Sep 13 00:51:04 PDT 2013

Op 13-09-13 09:46, Thomas Hellstrom schreef:
> On 09/13/2013 09:16 AM, Maarten Lankhorst wrote:
>> Op 13-09-13 08:44, Thomas Hellstrom schreef:
>>> 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 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.
>>> Actually, from looking at the mm code, it seems OK to do the following:
>>> if (!bo_tryreserve()) {
>>>      up_read mmap_sem(); // Release the mmap_sem to avoid deadlocks.
>>>      bo_reserve();               // Wait for the BO to become available (interruptible)
>>>      bo_unreserve();           // Where is bo_wait_unreserved() when we need it, Maarten :P
>>>      return VM_FAULT_RETRY; // Go ahead and retry the VMA walk, after regrabbing
>>> }
>> Is this meant as a jab at me? You're doing locking wrong here! Again!
> It's not meant as a jab at you.  I'm sorry if it came out that way. It was meant as a joke. I wasn't aware the topic was sensitive.
> Anyway, could you describe what is wrong, with the above solution, because it seems perfectly legal to me.
> There is no substantial overhead, and there is no risc of deadlocks. Or do you mean it's bad because it confuses lockdep?
Evil userspace can pass a bo as pointer to use for relocation lists, lockdep will warn when that locks up, but still..
This is already a problem now, and your fixing will only cause lockdep to explicitly warn on it.

You can make a complicated user program to test this, or simply use this function for debugging:
void ttm_might_fault(void) { struct reservation_object obj; reservation_object_init(&obj); ww_mutex_lock(&obj.lock, NULL); ww_mutex_unlock(&obj.lock); reservation_object_fini(&obj); }

Put it near every instance of copy_to_user/copy_from_user and you'll find the bugs. :)


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