drm/i915: Remove requirement for holding i915_request.lock for breadcrumbs

Since the breadcrumb enabling/cancelling itself is serialised by the
breadcrumbs.irq_lock, with a bit of care we can remove the outer
serialisation with i915_request.lock for concurrent
dma_fence_enable_signaling(). This has the important side-effect of
eliminating the nested i915_request.lock within request submission.

The challenge in serialisation is around the unsubmission where we take
an active request that wants a breadcrumb on the signaling engine and
put it to sleep. We do not want a concurrent
dma_fence_enable_signaling() to attach a breadcrumb as we unsubmit, so
we must mark the request as no longer active before serialising with the
concurrent enable-signaling.

On retire, we serialise with the concurrent enable-signaling, but
instead of clearing ACTIVE, we mark it as SIGNALED.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200731154834.8378-1-chris@chris-wilson.co.ukSigned-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
[Joonas: Rebased and reordered into drm-intel-gt-next branch]
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/gt/intel_breadcrumbs.c

@@ -220,17 +220,17 @@ static void signal_irq_work(struct irq_work *work)
 	}
 }
 
-static bool __intel_breadcrumbs_arm_irq(struct intel_breadcrumbs *b)
+static void __intel_breadcrumbs_arm_irq(struct intel_breadcrumbs *b)
 {
 	struct intel_engine_cs *engine =
 		container_of(b, struct intel_engine_cs, breadcrumbs);
 
 	lockdep_assert_held(&b->irq_lock);
 	if (b->irq_armed)
-		return true;
+		return;
 
 	if (!intel_gt_pm_get_if_awake(engine->gt))
-		return false;
+		return;
 
 	/*
 	 * The breadcrumb irq will be disarmed on the interrupt after the
@@ -250,8 +250,6 @@ static bool __intel_breadcrumbs_arm_irq(struct intel_breadcrumbs *b)
 
 	if (!b->irq_enabled++)
 		irq_enable(engine);
-
-	return true;
 }
 
 void intel_engine_init_breadcrumbs(struct intel_engine_cs *engine)
@@ -310,25 +308,16 @@ void intel_engine_fini_breadcrumbs(struct intel_engine_cs *engine)
 {
 }
 
-bool i915_request_enable_breadcrumb(struct i915_request *rq)
+static void insert_breadcrumb(struct i915_request *rq,
+			      struct intel_breadcrumbs *b)
 {
-	lockdep_assert_held(&rq->lock);
-
-	if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &rq->fence.flags))
-		return true;
-
-	if (test_bit(I915_FENCE_FLAG_ACTIVE, &rq->fence.flags)) {
-		struct intel_breadcrumbs *b = &rq->engine->breadcrumbs;
 	struct intel_context *ce = rq->context;
 	struct list_head *pos;
 
-		spin_lock(&b->irq_lock);
-
 	if (test_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags))
-			goto unlock;
+		return;
 
-		if (!__intel_breadcrumbs_arm_irq(b))
-			goto unlock;
+	__intel_breadcrumbs_arm_irq(b);
 
 	/*
 	 * We keep the seqno in retirement order, so we can break
@@ -357,10 +346,61 @@ bool i915_request_enable_breadcrumb(struct i915_request *rq)
 	GEM_BUG_ON(!check_signal_order(ce, rq));
 
 	set_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags);
-unlock:
+}
+
+bool i915_request_enable_breadcrumb(struct i915_request *rq)
+{
+	struct intel_breadcrumbs *b;
+
+	/* Serialises with i915_request_retire() using rq->lock */
+	if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &rq->fence.flags))
+		return true;
+
+	/*
+	 * Peek at i915_request_submit()/i915_request_unsubmit() status.
+	 *
+	 * If the request is not yet active (and not signaled), we will
+	 * attach the breadcrumb later.
+	 */
+	if (!test_bit(I915_FENCE_FLAG_ACTIVE, &rq->fence.flags))
+		return true;
+
+	/*
+	 * rq->engine is locked by rq->engine->active.lock. That however
+	 * is not known until after rq->engine has been dereferenced and
+	 * the lock acquired. Hence we acquire the lock and then validate
+	 * that rq->engine still matches the lock we hold for it.
+	 *
+	 * Here, we are using the breadcrumb lock as a proxy for the
+	 * rq->engine->active.lock, and we know that since the breadcrumb
+	 * will be serialised within i915_request_submit/i915_request_unsubmit,
+	 * the engine cannot change while active as long as we hold the
+	 * breadcrumb lock on that engine.
+	 *
+	 * From the dma_fence_enable_signaling() path, we are outside of the
+	 * request submit/unsubmit path, and so we must be more careful to
+	 * acquire the right lock.
+	 */
+	b = &READ_ONCE(rq->engine)->breadcrumbs;
+	spin_lock(&b->irq_lock);
+	while (unlikely(b != &READ_ONCE(rq->engine)->breadcrumbs)) {
 		spin_unlock(&b->irq_lock);
+		b = &READ_ONCE(rq->engine)->breadcrumbs;
+		spin_lock(&b->irq_lock);
 	}
 
+	/*
+	 * Now that we are finally serialised with request submit/unsubmit,
+	 * [with b->irq_lock] and with i915_request_retire() [via checking
+	 * SIGNALED with rq->lock] confirm the request is indeed active. If
+	 * it is no longer active, the breadcrumb will be attached upon
+	 * i915_request_submit().
+	 */
+	if (test_bit(I915_FENCE_FLAG_ACTIVE, &rq->fence.flags))
+		insert_breadcrumb(rq, b);
+
+	spin_unlock(&b->irq_lock);
+
 	return !__request_completed(rq);
 }
 
@@ -368,8 +408,6 @@ void i915_request_cancel_breadcrumb(struct i915_request *rq)
 {
 	struct intel_breadcrumbs *b = &rq->engine->breadcrumbs;
 
-	lockdep_assert_held(&rq->lock);
-
 	/*
 	 * We must wait for b->irq_lock so that we know the interrupt handler
 	 * has released its reference to the intel_context and has completed

drivers/gpu/drm/i915/gt/intel_lrc.c

@@ -1148,20 +1148,6 @@ __unwind_incomplete_requests(struct intel_engine_cs *engine)
 		} else {
 			struct intel_engine_cs *owner = rq->context->engine;
 
-			/*
-			 * Decouple the virtual breadcrumb before moving it
-			 * back to the virtual engine -- we don't want the
-			 * request to complete in the background and try
-			 * and cancel the breadcrumb on the virtual engine
-			 * (instead of the old engine where it is linked)!
-			 */
-			if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
-				     &rq->fence.flags)) {
-				spin_lock_nested(&rq->lock,
-						 SINGLE_DEPTH_NESTING);
-				i915_request_cancel_breadcrumb(rq);
-				spin_unlock(&rq->lock);
-			}
 			WRITE_ONCE(rq->engine, owner);
 			owner->submit_request(rq);
 			active = NULL;

drivers/gpu/drm/i915/i915_request.c

@@ -301,14 +301,24 @@ bool i915_request_retire(struct i915_request *rq)
 		dma_fence_signal_locked(&rq->fence);
 	if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &rq->fence.flags))
 		i915_request_cancel_breadcrumb(rq);
+	spin_unlock_irq(&rq->lock);
+
 	if (i915_request_has_waitboost(rq)) {
 		GEM_BUG_ON(!atomic_read(&rq->engine->gt->rps.num_waiters));
 		atomic_dec(&rq->engine->gt->rps.num_waiters);
 	}
-	if (!test_bit(I915_FENCE_FLAG_ACTIVE, &rq->fence.flags)) {
-		set_bit(I915_FENCE_FLAG_ACTIVE, &rq->fence.flags);
-		__notify_execute_cb(rq);
-	}
+
+	/*
+	 * We only loosely track inflight requests across preemption,
+	 * and so we may find ourselves attempting to retire a _completed_
+	 * request that we have removed from the HW and put back on a run
+	 * queue.
+	 *
+	 * As we set I915_FENCE_FLAG_ACTIVE on the request, this should be
+	 * after removing the breadcrumb and signaling it, so that we do not
+	 * inadvertently attach the breadcrumb to a completed request.
+	 */
+	remove_from_engine(rq);
 	GEM_BUG_ON(!llist_empty(&rq->execute_cb));
 	spin_unlock_irq(&rq->lock);
 
@@ -547,20 +557,22 @@ bool __i915_request_submit(struct i915_request *request)
 		clear_bit(I915_FENCE_FLAG_PQUEUE, &request->fence.flags);
 	}
 
-	/* We may be recursing from the signal callback of another i915 fence */
-	if (!i915_request_signaled(request)) {
-		spin_lock_nested(&request->lock, SINGLE_DEPTH_NESTING);
-
+	/*
+	 * XXX Rollback bonded-execution on __i915_request_unsubmit()?
+	 *
+	 * In the future, perhaps when we have an active time-slicing scheduler,
+	 * it will be interesting to unsubmit parallel execution and remove
+	 * busywaits from the GPU until their master is restarted. This is
+	 * quite hairy, we have to carefully rollback the fence and do a
+	 * preempt-to-idle cycle on the target engine, all the while the
+	 * master execute_cb may refire.
+	 */
 	__notify_execute_cb(request);
-		if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
-			     &request->fence.flags) &&
+
+	if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &request->fence.flags) &&
 	    !i915_request_enable_breadcrumb(request))
 		intel_engine_signal_breadcrumbs(engine);
 
-		spin_unlock(&request->lock);
-		GEM_BUG_ON(!llist_empty(&request->execute_cb));
-	}
-
 	return result;
 }
 
@@ -581,27 +593,27 @@ void __i915_request_unsubmit(struct i915_request *request)
 {
 	struct intel_engine_cs *engine = request->engine;
 
+	/*
+	 * Only unwind in reverse order, required so that the per-context list
+	 * is kept in seqno/ring order.
+	 */
 	RQ_TRACE(request, "\n");
 
 	GEM_BUG_ON(!irqs_disabled());
 	lockdep_assert_held(&engine->active.lock);
 
 	/*
-	 * Only unwind in reverse order, required so that the per-context list
-	 * is kept in seqno/ring order.
+	 * Before we remove this breadcrumb from the signal list, we have
+	 * to ensure that a concurrent dma_fence_enable_signaling() does not
+	 * attach itself. We first mark the request as no longer active and
+	 * make sure that is visible to other cores, and then remove the
+	 * breadcrumb if attached.
 	 */
-
-	/* We may be recursing from the signal callback of another i915 fence */
-	spin_lock_nested(&request->lock, SINGLE_DEPTH_NESTING);
-
+	GEM_BUG_ON(!test_bit(I915_FENCE_FLAG_ACTIVE, &request->fence.flags));
+	clear_bit_unlock(I915_FENCE_FLAG_ACTIVE, &request->fence.flags);
 	if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &request->fence.flags))
 		i915_request_cancel_breadcrumb(request);
 
-	GEM_BUG_ON(!test_bit(I915_FENCE_FLAG_ACTIVE, &request->fence.flags));
-	clear_bit(I915_FENCE_FLAG_ACTIVE, &request->fence.flags);
-
-	spin_unlock(&request->lock);
-
 	/* We've already spun, don't charge on resubmitting. */
 	if (request->sched.semaphores && i915_request_started(request))
 		request->sched.semaphores = 0;