drm/i915/gt: Pull marking vm as closed underneath the vm->mutex

Pull the final atomic_dec of vm->open (marking the vm as closed)
underneath the same vm->mutex as used to close it. This is required to
correctly serialise with attempting to reuse the vma as the vm is closed
by a second thread.

References: 00de702c ("drm/i915: Check that the vma hasn't been closed before we insert it")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200227085723.1961649-10-chris@chris-wilson.co.uk

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

@@ -171,7 +171,9 @@ void __i915_vm_close(struct i915_address_space *vm)
 {
 	struct i915_vma *vma, *vn;
 
-	mutex_lock(&vm->mutex);
+	if (!atomic_dec_and_mutex_lock(&vm->open, &vm->mutex))
+		return;
+
 	list_for_each_entry_safe(vma, vn, &vm->bound_list, vm_link) {
 		struct drm_i915_gem_object *obj = vma->obj;
 
@@ -186,6 +188,7 @@ void __i915_vm_close(struct i915_address_space *vm)
 		i915_gem_object_put(obj);
 	}
 	GEM_BUG_ON(!list_empty(&vm->bound_list));
+
 	mutex_unlock(&vm->mutex);
 }
 

drivers/gpu/drm/i915/gt/intel_gtt.h

@@ -429,8 +429,7 @@ static inline void
 i915_vm_close(struct i915_address_space *vm)
 {
 	GEM_BUG_ON(!atomic_read(&vm->open));
-	if (atomic_dec_and_test(&vm->open))
-		__i915_vm_close(vm);
+	__i915_vm_close(vm);
 
 	i915_vm_put(vm);
 }