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 17:24, 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..

Regards,

Tvrtko






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