Re: [PATCH 18/27] drm/i915/guc: Release submit fence from an irq_work

[Daniele Ceraolo Spurio <daniele.ceraolospurio@xxxxxxxxx>]

On 8/18/2021 11:16 PM, Matthew Brost wrote:
> A subsequent patch will flip the locking hierarchy from
> ce->guc_state.lock -> sched_engine->lock to sched_engine->lock ->
> ce->guc_state.lock. As such we need to release the submit fence for a
> request from an IRQ to break a lock inversion - i.e. the fence must be
> release went holding ce->guc_state.lock and the releasing of the can
> acquire sched_engine->lock.
>
> Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> ---
>   drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c | 15 ++++++++++++++-
>   drivers/gpu/drm/i915/i915_request.h               |  5 +++++
>   2 files changed, 19 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> index 9a53bae367b1..deb2e821e441 100644
> --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> @@ -2025,6 +2025,14 @@ static const struct intel_context_ops guc_context_ops = {
>   	.create_virtual = guc_create_virtual,
>   };
>   
> +static void submit_work_cb(struct irq_work *wrk)
> +{
> +	struct i915_request *rq = container_of(wrk, typeof(*rq), submit_work);
> +
> +	might_lock(&rq->engine->sched_engine->lock);
> +	i915_sw_fence_complete(&rq->submit);
> +}
> +
>   static void __guc_signal_context_fence(struct intel_context *ce)
>   {
>   	struct i915_request *rq;
> @@ -2034,8 +2042,12 @@ static void __guc_signal_context_fence(struct intel_context *ce)
>   	if (!list_empty(&ce->guc_state.fences))
>   		trace_intel_context_fence_release(ce);
>   
> +	/*
> +	 * Use an IRQ to ensure locking order of sched_engine->lock ->
> +	 * ce->guc_state.lock is preserved.
> +	 */
>   	list_for_each_entry(rq, &ce->guc_state.fences, guc_fence_link)
> -		i915_sw_fence_complete(&rq->submit);
> +		irq_work_queue(&rq->submit_work);

I think we should clear rq->guc_fence_link before queueing the work, 
just to make sure the work can't interfere back to this list (I know we 
don't now, it's just for future proofing paranoia). with that:

Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@xxxxxxxxx>

Daniele

>   
>   	INIT_LIST_HEAD(&ce->guc_state.fences);
>   }
> @@ -2145,6 +2157,7 @@ static int guc_request_alloc(struct i915_request *rq)
>   	spin_lock_irqsave(&ce->guc_state.lock, flags);
>   	if (context_wait_for_deregister_to_register(ce) ||
>   	    context_pending_disable(ce)) {
> +		init_irq_work(&rq->submit_work, submit_work_cb);
>   		i915_sw_fence_await(&rq->submit);
>   
>   		list_add_tail(&rq->guc_fence_link, &ce->guc_state.fences);
> diff --git a/drivers/gpu/drm/i915/i915_request.h b/drivers/gpu/drm/i915/i915_request.h
> index 1bc1349ba3c2..d818cfbfc41d 100644
> --- a/drivers/gpu/drm/i915/i915_request.h
> +++ b/drivers/gpu/drm/i915/i915_request.h
> @@ -218,6 +218,11 @@ struct i915_request {
>   	};
>   	struct llist_head execute_cb;
>   	struct i915_sw_fence semaphore;
> +	/**
> +	 * @submit_work: complete submit fence from an IRQ if needed for
> +	 * locking hierarchy reasons.
> +	 */
> +	struct irq_work submit_work;
>   
>   	/*
>   	 * A list of everyone we wait upon, and everyone who waits upon us.