drm/i915/guc: Extend deregistration fence to schedule disable

Extend the deregistration context fence to fence whne a GuC context has
scheduling disable pending.

v2:
 (John H)
  - Update comment why we check the pin count within spin lock

Cc: John Harrison <john.c.harrison@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <john.c.harrison@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210721215101.139794-11-matthew.brost@intel.com

drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c

@@ -930,7 +930,22 @@ static void guc_context_sched_disable(struct intel_context *ce)
 		goto unpin;
 
 	spin_lock_irqsave(&ce->guc_state.lock, flags);
+
+	/*
+	 * We have to check if the context has been pinned again as another pin
+	 * operation is allowed to pass this function. Checking the pin count,
+	 * within ce->guc_state.lock, synchronizes this function with
+	 * guc_request_alloc ensuring a request doesn't slip through the
+	 * 'context_pending_disable' fence. Checking within the spin lock (can't
+	 * sleep) ensures another process doesn't pin this context and generate
+	 * a request before we set the 'context_pending_disable' flag here.
+	 */
+	if (unlikely(atomic_add_unless(&ce->pin_count, -2, 2))) {
+		spin_unlock_irqrestore(&ce->guc_state.lock, flags);
+		return;
+	}
 	guc_id = prep_context_pending_disable(ce);
+
 	spin_unlock_irqrestore(&ce->guc_state.lock, flags);
 
 	with_intel_runtime_pm(runtime_pm, wakeref)
@@ -1135,19 +1150,22 @@ static int guc_request_alloc(struct i915_request *rq)
 out:
 	/*
 	 * We block all requests on this context if a G2H is pending for a
-	 * context deregistration as the GuC will fail a context registration
-	 * while this G2H is pending. Once a G2H returns, the fence is released
-	 * that is blocking these requests (see guc_signal_context_fence).
+	 * schedule disable or context deregistration as the GuC will fail a
+	 * schedule enable or context registration if either G2H is pending
+	 * respectfully. Once a G2H returns, the fence is released that is
+	 * blocking these requests (see guc_signal_context_fence).
 	 *
-	 * We can safely check the below field outside of the lock as it isn't
-	 * possible for this field to transition from being clear to set but
+	 * We can safely check the below fields outside of the lock as it isn't
+	 * possible for these fields to transition from being clear to set but
 	 * converse is possible, hence the need for the check within the lock.
 	 */
-	if (likely(!context_wait_for_deregister_to_register(ce)))
+	if (likely(!context_wait_for_deregister_to_register(ce) &&
+		   !context_pending_disable(ce)))
 		return 0;
 
 	spin_lock_irqsave(&ce->guc_state.lock, flags);
-	if (context_wait_for_deregister_to_register(ce)) {
+	if (context_wait_for_deregister_to_register(ce) ||
+	    context_pending_disable(ce)) {
 		i915_sw_fence_await(&rq->submit);
 
 		list_add_tail(&rq->guc_fence_link, &ce->guc_state.fences);
@@ -1491,10 +1509,18 @@ int intel_guc_sched_done_process_msg(struct intel_guc *guc,
 	if (context_pending_enable(ce)) {
 		clr_context_pending_enable(ce);
 	} else if (context_pending_disable(ce)) {
+		/*
+		 * Unpin must be done before __guc_signal_context_fence,
+		 * otherwise a race exists between the requests getting
+		 * submitted + retired before this unpin completes resulting in
+		 * the pin_count going to zero and the context still being
+		 * enabled.
+		 */
 		intel_context_sched_disable_unpin(ce);
 
 		spin_lock_irqsave(&ce->guc_state.lock, flags);
 		clr_context_pending_disable(ce);
+		__guc_signal_context_fence(ce);
 		spin_unlock_irqrestore(&ce->guc_state.lock, flags);
 	}