Re: [PATCH 4/4] locking/locktorture: Fix incorrect use of ww_acquire_ctx in ww_mutex test

[Davidlohr Bueso]

On Tue, 16 Mar 2021, Waiman Long wrote:

> The ww_acquire_ctx structure for ww_mutex needs to persist for a complete
> lock/unlock cycle. In the ww_mutex test in locktorture, however, both
> ww_acquire_init() and ww_acquire_fini() are called within the lock
> function only. This causes a lockdep splat of "WARNING: Nested lock
> was not taken" when lockdep is enabled in the kernel.
>
> To fix this problem, we need to move the ww_acquire_fini() after the
> ww_mutex_unlock() in torture_ww_mutex_unlock(). In other word, we need
> to pass state information from the lock function to the unlock function.

Right, and afaict this _is_ the way ww_acquire_fini() should be called:

 * Releases a w/w acquire context. This must be called _after_ all acquired w/w
 * mutexes have been released with ww_mutex_unlock.

> Change the writelock and writeunlock function prototypes to allow that
> and change the torture_ww_mutex_lock() and torture_ww_mutex_unlock()
> accordingly.

But wouldn't just making ctx a global variable be enough instead? That way
we don't deal with memory allocation for every lock/unlock operation (yuck).
Plus the ENOMEM would need to be handled/propagated accordingly - the code
really doesn't expect any failure from ->writelock().

diff --git a/kernel/locking/locktorture.c b/kernel/locking/locktorture.c
index 0ab94e1f1276..606c0f6c1657 100644
--- a/kernel/locking/locktorture.c
+++ b/kernel/locking/locktorture.c
@@ -362,6 +362,8 @@ static DEFINE_WW_MUTEX(torture_ww_mutex_0, &torture_ww_class);
 static DEFINE_WW_MUTEX(torture_ww_mutex_1, &torture_ww_class);
 static DEFINE_WW_MUTEX(torture_ww_mutex_2, &torture_ww_class);

+static struct ww_acquire_ctx ctx;
+
 static int torture_ww_mutex_lock(void)
 __acquires(torture_ww_mutex_0)
 __acquires(torture_ww_mutex_1)
@@ -372,7 +374,6 @@ __acquires(torture_ww_mutex_2)
		struct list_head link;
		struct ww_mutex *lock;
	} locks[3], *ll, *ln;
-	struct ww_acquire_ctx ctx;

	locks[0].lock = &torture_ww_mutex_0;
	list_add(&locks[0].link, &list);
@@ -403,7 +404,6 @@ __acquires(torture_ww_mutex_2)
		list_move(&ll->link, &list);
	}

-	ww_acquire_fini(&ctx);
	return 0;
 }

@@ -415,6 +415,8 @@ __releases(torture_ww_mutex_2)
	ww_mutex_unlock(&torture_ww_mutex_0);
	ww_mutex_unlock(&torture_ww_mutex_1);
	ww_mutex_unlock(&torture_ww_mutex_2);
+
+	ww_acquire_fini(&ctx);
 }

 static struct lock_torture_ops ww_mutex_lock_ops = {