Quoting Chris Wilson (2019-11-18 16:23:42)
> In commit a79ca656b648 ("drm/i915: Push the wakeref->count deferral to
> the backend"), I erroneously concluded that we last modify the engine
> inside __i915_request_commit() meaning that we could enable concurrent
> submission for userspace as we enqueued this request. However, this
> falls into a trap with other users of the engine->kernel_context waking
> up and submitting their request before the idle-switch is queued, with
> the result that the kernel_context is executed out-of-sequence most
> likely upsetting the GPU and certainly ourselves when we try to retire
> the out-of-sequence requests.
>
> As such we need to hold onto the effective engine->kernel_context mutex
> lock (via the engine pm mutex proxy) until we have finish queuing the
> request to the engine.
>
> Fixes: a79ca656b648 ("drm/i915: Push the wakeref->count deferral to the
> backend")
> Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuopp...@linux.intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursu...@intel.com>
> ---
> drivers/gpu/drm/i915/gt/intel_engine_pm.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> index 3c0f490ff2c7..2d2a21752ae4 100644
> --- a/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> +++ b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> @@ -116,11 +116,12 @@ static bool switch_to_kernel_context(struct
> intel_engine_cs *engine)
> rq->sched.attr.priority = I915_PRIORITY_BARRIER;
> __i915_request_commit(rq);
>
> - /* Release our exclusive hold on the engine */
> - __intel_wakeref_defer_park(&engine->wakeref);
> __i915_request_queue(rq, NULL);
>
> + /* Release our exclusive hold on the engine */
> + __intel_wakeref_defer_park(&engine->wakeref);
> result = false;
Gah, now I remember why I put it before:
if there is a concurrent retire requests, it may now see the request
completion prior to us marking the engine as awake => counter underflow.
Watch this space for more tricks :(
-Chris
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