[Intel-gfx] [PATCH 4/7] drm/i915: Delay the freeing of requests until retire time

[John Harrison]

On 08/01/2016 22:08, Chris Wilson wrote:
> On Fri, Jan 08, 2016 at 06:47:25PM +0000, John.C.Harrison at Intel.com wrote:
>> From: John Harrison <John.C.Harrison at Intel.com>
>>
>> The request structure is reference counted. When the count reached
>> zero, the request was immediately freed and all associated objects
>> were unrefereced/unallocated. This meant that the driver mutex lock
>> must be held at the point where the count reaches zero. This was fine
>> while all references were held internally to the driver. However, the
>> plan is to allow the underlying fence object (and hence the request
>> itself) to be returned to other drivers and to userland. External
>> users cannot be expected to acquire a driver private mutex lock.
> It's a trivial issue to fix to enable freeing requests without holding the
> struct_mutex. You don't need to even add any new lists, delayed freeing
> mechanisms and whotnot.
> -Chris
>

As the driver stands, it is not trivial to free a request without 
holding the mutex. It does things like unpinning buffers, freeing up 
contexts (which is a whole other bundle of complication), releasing 
IRQs. It may be possible to re-organise things to make those operations 
safe to do without the mutex but it certainly does not look trivial!