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

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@intel.com

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@intel.com

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.