linux-kernel - [RFC][PATCH] locking: Fix __clear_task_blocked_on() warning from __ww_mutex

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Message-ID: <20250731201337.274382-1-jstultz@google.com>
Date: Thu, 31 Jul 2025 20:13:22 +0000
From: John Stultz <jstultz@...gle.com>
To: LKML <linux-kernel@...r.kernel.org>
Cc: John Stultz <jstultz@...gle.com>, 
	syzbot+602c4720aed62576cd79@...kaller.appspotmail.com, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, 
	Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>, 
	Valentin Schneider <valentin.schneider@....com>, K Prateek Nayak <kprateek.nayak@....com>, 
	Suleiman Souhlal <suleiman@...gle.com>, airlied@...il.com, mripard@...nel.org, 
	simona@...ll.ch, tzimmermann@...e.de, dri-devel@...ts.freedesktop.org, 
	kernel-team@...roid.com
Subject: [RFC][PATCH] locking: Fix __clear_task_blocked_on() warning from
 __ww_mutex_wound() path

The __clear_task_blocked_on() helper added a number of sanity
checks ensuring we hold the lock and that the task we are
clearing blocked_on pointer (if set) matches the lock.

However, there is an edge case in the _ww_mutex_wound() logic
where we need to clear the blocked_on pointer for the task that
owns the lock, not the task that is waiting on the lock.

For this case the sanity checks aren't valid, so handle this
by allowing a NULL lock to skip the additional checks.

This was easier to miss, I realized, as the test-ww_mutex
driver only exercises the wait-die class of ww_mutexes.

I've got a follow up patch to extend the test so that it
will exercise both.

Fixes: a4f0b6fef4b0 ("locking/mutex: Add p->blocked_on wrappers for correctness checks")
Reported-by: syzbot+602c4720aed62576cd79@...kaller.appspotmail.com
Reported-by: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
Closes: https://lore.kernel.org/lkml/68894443.a00a0220.26d0e1.0015.GAE@google.com/
Signed-off-by: John Stultz <jstultz@...gle.com>
---
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Valentin Schneider <valentin.schneider@....com>
Cc: K Prateek Nayak <kprateek.nayak@....com>
Cc: Suleiman Souhlal <suleiman@...gle.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
Cc: airlied@...il.com
Cc: mripard@...nel.org
Cc: simona@...ll.ch
Cc: tzimmermann@...e.de
Cc: dri-devel@...ts.freedesktop.org
Cc: kernel-team@...roid.com
---
 include/linux/sched.h     | 19 ++++++++++---------
 kernel/locking/ww_mutex.h |  6 +++++-
 2 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 40d2fa90df425..a9a78f51f7f57 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2166,15 +2166,16 @@ static inline void set_task_blocked_on(struct task_struct *p, struct mutex *m)
 
 static inline void __clear_task_blocked_on(struct task_struct *p, struct mutex *m)
 {
-	WARN_ON_ONCE(!m);
-	/* Currently we serialize blocked_on under the mutex::wait_lock */
-	lockdep_assert_held_once(&m->wait_lock);
-	/*
-	 * There may be cases where we re-clear already cleared
-	 * blocked_on relationships, but make sure we are not
-	 * clearing the relationship with a different lock.
-	 */
-	WARN_ON_ONCE(m && p->blocked_on && p->blocked_on != m);
+	if (m) {
+		/* Currently we serialize blocked_on under the mutex::wait_lock */
+		lockdep_assert_held_once(&m->wait_lock);
+		/*
+		 * There may be cases where we re-clear already cleared
+		 * blocked_on relationships, but make sure we are not
+		 * clearing the relationship with a different lock.
+		 */
+		WARN_ON_ONCE(m && p->blocked_on && p->blocked_on != m);
+	}
 	p->blocked_on = NULL;
 }
 
diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
index 086fd5487ca77..ef8ef3c28592c 100644
--- a/kernel/locking/ww_mutex.h
+++ b/kernel/locking/ww_mutex.h
@@ -342,8 +342,12 @@ static bool __ww_mutex_wound(struct MUTEX *lock,
 			 * When waking up the task to wound, be sure to clear the
 			 * blocked_on pointer. Otherwise we can see circular
 			 * blocked_on relationships that can't resolve.
+			 *
+			 * NOTE: We pass NULL here instead of lock, because we
+			 * are waking the lock owner, who may be currently blocked
+			 * on a different lock.
 			 */
-			__clear_task_blocked_on(owner, lock);
+			__clear_task_blocked_on(owner, NULL);
 			wake_q_add(wake_q, owner);
 		}
 		return true;
-- 
2.50.1.565.gc32cd1483b-goog