linux-kernel - Re: [PATCH 4/4] locking/locktorture: Fix incorrect use of ww_acquire_ctx in ww

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Date:   Tue, 16 Mar 2021 22:16:05 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        linux-kernel@...r.kernel.org, Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH 4/4] locking/locktorture: Fix incorrect use of
 ww_acquire_ctx in ww_mutex test

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 = {