[BUG] completely bonkers use of set_need_resched + VM_FAULT_NOPAGE

Maarten Lankhorst maarten.lankhorst at canonical.com
Thu Sep 12 08:45:17 PDT 2013

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..
> The one in udl just looks like copypasta from i915, without any
> justification (at least I don't see any) for why it's there. Probably
> can die, too, since there isn't any gpu to reset on usb display-link
> devices ...
> Cheers, Daniel

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