Re: [BrownBag] drm/i915/gtt: Neuter the deferred unbind callback from gen6_ppgtt_cleanup

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 24/05/2019 09:29, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2019-05-24 09:23:40)
>>
>> On 24/05/2019 09:17, Chris Wilson wrote:
>>> Quoting Tvrtko Ursulin (2019-05-24 09:13:14)
>>>>
>>>> On 24/05/2019 07:45, Chris Wilson wrote:
>>>>> Having deferred the vma destruction to a worker where we can acquire the
>>>>> struct_mutex, we have to avoid chasing back into the now destroyed
>>>>> ppgtt. The pd_vma is special in having a custom unbind function to scan
>>>>> for unused pages despite the VMA itself being notionally part of the
>>>>> GGTT. As such, we need to disable that callback to avoid a
>>>>> use-after-free.
>>>>>
>>>>> This unfortunately blew up so early during boot that CI declared the
>>>>> machine unreachable as opposed to being the major failure it was. Oops.
>>>>>
>>>>> Fixes: d3622099c76f ("drm/i915/gtt: Always acquire struct_mutex for gen6_ppgtt_cleanup")
>>>>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>>>>> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>>> Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
>>>>> ---
>>>>>     drivers/gpu/drm/i915/i915_gem_gtt.c | 28 ++++++++++++++++++++++++++++
>>>>>     1 file changed, 28 insertions(+)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
>>>>> index 8d8a4b0ad4d9..266baa11df64 100644
>>>>> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
>>>>> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
>>>>> @@ -1847,6 +1847,33 @@ static void gen6_ppgtt_cleanup_work(struct work_struct *wrk)
>>>>>         kfree(work);
>>>>>     }
>>>>>     
>>>>> +static int nop_set_pages(struct i915_vma *vma)
>>>>> +{
>>>>> +     return -ENODEV;
>>>>> +}
>>>>> +
>>>>> +static void nop_clear_pages(struct i915_vma *vma)
>>>>> +{
>>>>> +}
>>>>> +
>>>>> +static int nop_bind(struct i915_vma *vma,
>>>>> +                 enum i915_cache_level cache_level,
>>>>> +                 u32 unused)
>>>>> +{
>>>>> +     return -ENODEV;
>>>>> +}
>>>>> +
>>>>> +static void nop_unbind(struct i915_vma *vma)
>>>>> +{
>>>>> +}
>>>>> +
>>>>> +static const struct i915_vma_ops nop_vma_ops = {
>>>>> +     .set_pages = nop_set_pages,
>>>>> +     .clear_pages = nop_clear_pages,
>>>>> +     .bind_vma = nop_bind,
>>>>> +     .unbind_vma = nop_unbind,
>>>>> +};
>>>>> +
>>>>>     static void gen6_ppgtt_cleanup(struct i915_address_space *vm)
>>>>>     {
>>>>>         struct gen6_hw_ppgtt *ppgtt = to_gen6_ppgtt(i915_vm_to_ppgtt(vm));
>>>>> @@ -1855,6 +1882,7 @@ static void gen6_ppgtt_cleanup(struct i915_address_space *vm)
>>>>>         /* FIXME remove the struct_mutex to bring the locking under control */
>>>>>         INIT_WORK(&work->base, gen6_ppgtt_cleanup_work);
>>>>>         work->vma = ppgtt->vma;
>>>>> +     work->vma->ops = &nop_vma_ops;
>>>>
>>>> Could we use some asserts before overriding the vma ops? Like
>>>> GEM_BUG_ON(vma->pages)? And something for still bound?
>>>
>>> It technically still is bound as it is in the GGTT but currently
>>> unpinned -- that will be checked on destroy, it's just we also get an
>>> unbind callback. vma->pages doesn't exist for this (set to ERR_PTR).
>>
>> If we are getting the unbind callback and we nop-ed it, who will
>> actually do it's job?
> 
> The callback is just a hook for us to prune within the ppgtt.
> It still is removed from GGTT by i915_vma_unbind().

So it needs GEM_BUG_ON(ppgtt->scan_for_unused_pt) before overriding the 
unbind?

Regards,

Tvrtko
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