drm/xe: Make exec_queue_kill safe to call twice

An upcoming PXP patch will kill queues at runtime when a PXP
invalidation event occurs, so we need exec_queue_kill to be safe to call
multiple times.

v2: Add documentation (Matt B)
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240814205654.1716586-1-daniele.ceraolospurio@intel.com

drivers/gpu/drm/xe/xe_exec_queue.c

@@ -770,6 +770,15 @@ void xe_exec_queue_update_run_ticks(struct xe_exec_queue *q)
 	xef->run_ticks[q->class] += (new_ts - old_ts) * q->width;
 }
 
+/**
+ * xe_exec_queue_kill - permanently stop all execution from an exec queue
+ * @q: The exec queue
+ *
+ * This function permanently stops all activity on an exec queue. If the queue
+ * is actively executing on the HW, it will be kicked off the engine; any
+ * pending jobs are discarded and all future submissions are rejected.
+ * This function is safe to call multiple times.
+ */
 void xe_exec_queue_kill(struct xe_exec_queue *q)
 {
 	struct xe_exec_queue *eq = q, *next;

drivers/gpu/drm/xe/xe_vm.c

@@ -275,6 +275,8 @@ int xe_vm_add_compute_exec_queue(struct xe_vm *vm, struct xe_exec_queue *q)
  * xe_vm_remove_compute_exec_queue() - Remove compute exec queue from VM
  * @vm: The VM.
  * @q: The exec_queue
+ *
+ * Note that this function might be called multiple times on the same queue.
  */
 void xe_vm_remove_compute_exec_queue(struct xe_vm *vm, struct xe_exec_queue *q)
 {
@@ -282,8 +284,10 @@ void xe_vm_remove_compute_exec_queue(struct xe_vm *vm, struct xe_exec_queue *q)
 		return;
 
 	down_write(&vm->lock);
-	list_del(&q->lr.link);
-	--vm->preempt.num_exec_queues;
+	if (!list_empty(&q->lr.link)) {
+		list_del_init(&q->lr.link);
+		--vm->preempt.num_exec_queues;
+	}
 	if (q->lr.pfence) {
 		dma_fence_enable_sw_signaling(q->lr.pfence);
 		dma_fence_put(q->lr.pfence);