Re: [PATCH v3] drm/i915: Ensure associated VMAs are inactive when contexts are destroyed

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

On 17/11/15 17:32, Tvrtko Ursulin wrote:
>
> On 17/11/15 17:24, Tvrtko Ursulin wrote:
>>
>> On 17/11/15 17:08, Daniel Vetter wrote:
>>> On Tue, Nov 17, 2015 at 04:54:50PM +0000, Tvrtko Ursulin wrote:
>>>>
>>>> On 17/11/15 16:39, Daniel Vetter wrote:
>>>>> On Tue, Nov 17, 2015 at 04:27:12PM +0000, Tvrtko Ursulin wrote:
>>>>>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>>>>
>>>>>> In the following commit:
>>>>>>
>>>>>>      commit e9f24d5fb7cf3628b195b18ff3ac4e37937ceeae
>>>>>>      Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>>>>      Date:   Mon Oct 5 13:26:36 2015 +0100
>>>>>>
>>>>>>          drm/i915: Clean up associated VMAs on context destruction
>>>>>>
>>>>>> I added a WARN_ON assertion that VM's active list must be empty
>>>>>> at the time of owning context is getting freed, but that turned
>>>>>> out to be a wrong assumption.
>>>>>>
>>>>>> Due ordering of operations in i915_gem_object_retire__read, where
>>>>>> contexts are unreferenced before VMAs are moved to the inactive
>>>>>> list, the described situation can in fact happen.
>>>>>>
>>>>>> It feels wrong to do things in such order so this fix makes sure
>>>>>> a reference to context is held until the move to inactive list
>>>>>> is completed.
>>>>>>
>>>>>> v2: Rather than hold a temporary context reference move the
>>>>>>      request unreference to be the last operation. (Daniel Vetter)
>>>>>>
>>>>>> v3: Fix use after free. (Chris Wilson)
>>>>>>
>>>>>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>>>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92638
>>>>>> Cc: Michel Thierry <michel.thierry@intel.com>
>>>>>> Cc: Chris Wilson <chris@chris-wilson.co.uk>
>>>>>> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
>>>>>> ---
>>>>>>   drivers/gpu/drm/i915/i915_gem.c | 33
>>>>>> ++++++++++++++++++---------------
>>>>>>   1 file changed, 18 insertions(+), 15 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/i915/i915_gem.c
>>>>>> b/drivers/gpu/drm/i915/i915_gem.c
>>>>>> index 98c83286ab68..094ac17a712d 100644
>>>>>> --- a/drivers/gpu/drm/i915/i915_gem.c
>>>>>> +++ b/drivers/gpu/drm/i915/i915_gem.c
>>>>>> @@ -2404,29 +2404,32 @@ i915_gem_object_retire__read(struct
>>>>>> drm_i915_gem_object *obj, int ring)
>>>>>>       RQ_BUG_ON(!(obj->active & (1 << ring)));
>>>>>>
>>>>>>       list_del_init(&obj->ring_list[ring]);
>>>>>> -    i915_gem_request_assign(&obj->last_read_req[ring], NULL);
>>>>>>
>>>>>>       if (obj->last_write_req && obj->last_write_req->ring->id ==
>>>>>> ring)
>>>>>>           i915_gem_object_retire__write(obj);
>>>>>>
>>>>>>       obj->active &= ~(1 << ring);
>>>>>> -    if (obj->active)
>>>>>> -        return;
>>>>>
>>>>>     if (obj->active) {
>>>>>         i915_gem_request_assign(&obj->last_read_req[ring], NULL);
>>>>>         return;
>>>>>     }
>>>>>
>>>>> Would result in less churn in the code and drop the unecessary indent
>>>>> level. Also comment is missing as to why we need to do things in a
>>>>> specific order.
>>>>
>>>> Actually I think I changed my mind and that v1 is the way to go.
>>>>
>>>> Just re-ordering the code here still makes it possible for the context
>>>> destructor to run with VMAs on the active list I think.
>>>>
>>>> If we hold the context then it is 100% clear it is not possible.
>>>
>>> request_assign _is_ the function which adjust the refcounts for us,
>>> which
>>> means if we drop that reference too early then grabbing a temp reference
>>> is just papering over the real bug.
>>>
>>> Written out your patch looks something like
>>>
>>>     a_reference(a);
>>>     a_unreference(a);
>>>
>>>     /* more cleanup code that should get run before a_unreference but
>>> isn't */
>>>
>>>     a_unrefernce(a); /* for real this time */
>>>
>>> Unfortunately foo_assign is a new pattern and not well-established, so
>>> that connection isn't clear. Maybe we should rename it to
>>> foo_reference_assign to make it more obvious. Or just drop the pretense
>>> and open-code it since we unconditionally assign NULL as the new pointer
>>> value, and we know the current value of the pointer is non-NULL. So
>>> there's really no benefit to the helper here, it only obfuscates. And
>>> since that obfuscation tripped you up it's time to remove it ;-)
>>
>> Then foo_reference_unreference_assign. :)
>>
>> But seriously, I think it is more complicated that..
>>
>> The thing it trips over is that moving VMAs to inactive does not
>> correspond in time to request retirement. But in fact VMAs are moved to
>> inactive only when all requests associated with an object are done.
>>
>> This is the unintuitive thing I was working around. To make sure when
>> context destructor runs there are not active VMAs for that VM.
>>
>> I don't know how to guarantee that with what you propose. Perhaps I am
>> missing something?
>
> Maybe completely different approach would be to find the VMA belonging
> to req->ctx->vm in i915_gem_request_free and move it to the inactive
> list before context unreference there?
>
> Should would I think, if request is going away means VMA definitely
> needs to go to inactive..

Or not.. no link to the correct object from there. Ok calling it a day..

Tvrtko

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