[Intel-gfx] [PATCH] Revert "drm/i915: Skip execlists_dequeue() early if the list is empty"

[Chris Wilson]

On Wed, Mar 29, 2017 at 11:00:52AM +0100, Chris Wilson wrote:
> This reverts commit 6c943de6686f ("drm/i915: Skip execlists_dequeue()
> early if the list is empty").
> 
> The validity of using READ_ONCE there depends upon having a mb to
> coordinate the assignment of engine->execlist_first inside
> submit_request() and checking prior to taking the spinlock in
> execlists_dequeue(). We wrote "the update to TASKLET_SCHED incurs a
> memory barrier making this cross-cpu checking safe", but failed to
> notice that this mb was *conditional* on the execlists being ready, i.e.
> there wasn't the request mb when it was most necessary!

s/request/required/

> We could install an unconditional memory barrier to fixup the
> READ_ONCE():
> 
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c
> b/drivers/gpu/drm/i915/intel_lrc.c
> index 7dd732cb9f57..1ed164b16d44 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -616,6 +616,7 @@ static void execlists_submit_request(struct
> drm_i915_gem_request *request)
> 
>         if (insert_request(&request->priotree, &engine->execlist_queue))
> {
>                 engine->execlist_first = &request->priotree.node;
> +               smp_wmb();
>                 if (execlists_elsp_ready(engine))
> 
> But we have opted to remove the race as it should be rarely effective,
> and saves us having to explain the necessary memory barriers which we
> quite clearly failed at.
> 

Reported-and-tested-by: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> Fixes: 6c943de6686f ("drm/i915: Skip execlists_dequeue() early if the list is empty")
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> Cc: Michał Winiarski <michal.winiarski at intel.com>
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre