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 17:08, 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?

Regards,

Tvrtko


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