[BUG] completely bonkers use of set_need_resched + VM_FAULT_NOPAGE

Thomas Hellstrom thellstrom at vmware.com
Thu Sep 12 09:33:33 PDT 2013

On 09/12/2013 05:58 PM, Daniel Vetter wrote:
> On Thu, Sep 12, 2013 at 5:43 PM, Peter Zijlstra <peterz at infradead.org> wrote:
>>> 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.

Could you describe how it could recurse into it's own pagefault handler?
IIRC the VM flags of the TTM VMAs makes get_user_pages() refrain from 
touching these VMAs,
hence I don't think this code can deadlock, but admittedly it's far from 
the optimal solution.

Never mind, more on the set_need_resched() below.

>>>   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 ;-)
>> Yikes.. so how common is it? If I simply rip the set_need_resched() out
>> it will 'spin' on the fault a little longer until a 'natural' preemption
>> point -- if such a thing is every going to happen.

A typical case is if a process is throwing out a buffer from the GPU or 
system memory while another
process pagefaults while writing to it. It's not a common situation, and 
it's by no means a fastpath situation.
For correctness purposes, I think set_need_resched() can be safely removed.

> It's a case of "our userspace doesn't do this", so as long as you're
> not evil and frob the drm device nodes of ttm drivers directly the
> deadlock will never happen. No idea how much contention actually
> happens on e.g. shared buffer objects - in i915 we have just one lock
> and so suffer quite a bit more from contention. So no idea how much
> removing the yield would hurt.
> -Daniel


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