kunit: Make kunit_remove_resource() idempotent

The kunit_remove_resource() function is used to unlink a resource from
the list of resources in the test, making it no longer show up in
kunit_find_resource().

However, this could lead to a race condition if two threads called
kunit_remove_resource() on the same resource at the same time: the
resource would be removed from the list twice (causing a crash at the
second list_del()), and the refcount for the resource would be
decremented twice (instead of once, for the reference held by the
resource list).

Fix both problems, the first by using list_del_init(), and the second by
checking if the resource has already been removed using list_empty(),
and only decrementing its refcount if it has not.

Also add a KUnit test for the kunit_remove_resource() function which
tests this behaviour.

Reported-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

1 files changed, 6 insertions, 2 deletions

diff --git a/lib/kunit/resource.c b/lib/kunit/resource.c
index 8f8057aad78f..b3ba0d98d89e 100644
--- a/lib/kunit/resource.c
+++ b/lib/kunit/resource.c
@@ -98,11 +98,15 @@ EXPORT_SYMBOL_GPL(kunit_alloc_and_get_resource);
 void kunit_remove_resource(struct kunit *test, struct kunit_resource *res)
 {
 	unsigned long flags;
+	bool was_linked;
 
 	spin_lock_irqsave(&test->lock, flags);
-	list_del(&res->node);
+	was_linked = !list_empty(&res->node);
+	list_del_init(&res->node);
 	spin_unlock_irqrestore(&test->lock, flags);
-	kunit_put_resource(res);
+
+	if (was_linked)
+		kunit_put_resource(res);
 }
 EXPORT_SYMBOL_GPL(kunit_remove_resource);