[Intel-gfx] [PATCH v6 4/7] drm/i915: Delay the freeing of requests until retire time

[Maarten Lankhorst]

Hey,

Op 18-02-16 om 15:24 schreef John.C.Harrison at Intel.com:
> 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.
>
> Rather than attempt to disentangle the request structure from the
> driver mutex lock, the decsion was to defer the free code until a
> later (safer) point. Hence this patch changes the unreference callback
> to merely move the request onto a delayed free list. The driver's
> retire worker thread will then process the list and actually call the
> free function on the requests.
>
> v2: New patch in series.
>
> v3: Updated after review comments by Tvrtko Ursulin. Rename list nodes
> to 'link' rather than 'list'. Update list processing to be more
> efficient/safer with respect to spinlocks.
>
> v4: Changed to use basic spinlocks rather than IRQ ones - missed
> update from earlier feedback by Tvrtko.
>
> v5: Improved a comment to keep the style checker happy.
>
> For: VIZ-5190
> Signed-off-by: John Harrison <John.C.Harrison at Intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Looks like Chris also mentioned it, but a fence can stay alive for an unknown period of time.

As a result when a fence is signaled all associated data should be freed as soon as the fence is signaled,
not when the last refcount is dropped to 0. This will remove the delayed free dance and clean up code. :)

~Maarten