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In commita79ca656b6("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. v2: Serialise against concurrent intel_gt_retire_requests() v3: Describe the hairy locking scheme with intel_gt_retire_requests() for future reference. v4: Combine timeline->lock and engine pm release; it's hairy. Fixes:a79ca656b6("drm/i915: Push the wakeref->count deferral to the backend") Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20191120165514.3955081-2-chris@chris-wilson.co.uk (cherry picked from commit5cba288466) Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>