linux-kernel - [patch 47/50] rtmutex: Prevent lockdep false positive with PI futexes

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Message-Id: <20210713160750.564432923@linutronix.de>
Date:   Tue, 13 Jul 2021 17:11:41 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: [patch 47/50] rtmutex: Prevent lockdep false positive with PI futexes

From: Thomas Gleixner <tglx@...utronix.de>

On PREEMPT_RT the futex hashbucket spinlock becomes 'sleeping' and rtmutex
based. That causes a lockdep false positive because some of the futex
functions invoke spin_unlock(&hb->lock) with the wait_lock of the rtmutex
associated to the pi_futex held.  spin_unlock() in turn takes wait_lock of
the rtmutex on which the spinlock is based which makes lockdep notice a
lock recursion.

Give the futex/rtmutex wait_lock a seperate key.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/locking/rtmutex_api.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)
---
--- a/kernel/locking/rtmutex_api.c
+++ b/kernel/locking/rtmutex_api.c
@@ -209,7 +209,19 @@ EXPORT_SYMBOL_GPL(__rt_mutex_init);
 void __sched rt_mutex_init_proxy_locked(struct rt_mutex *lock,
 					struct task_struct *proxy_owner)
 {
+	static struct lock_class_key pi_futex_key;
+
 	__rt_mutex_basic_init(lock);
+	/*
+	 * On PREEMPT_RT the futex hashbucket spinlock becomes 'sleeping'
+	 * and rtmutex based. That causes a lockdep false positive because
+	 * some of the futex functions invoke spin_unlock(&hb->lock) with
+	 * the wait_lock of the rtmutex associated to the pi_futex held.
+	 * spin_unlock() in turn takes wait_lock of the rtmutex on which
+	 * the spinlock is based which makes lockdep notice a lock
+	 * recursion. Give the futex/rtmutex wait_lock a seperate key.
+	 */
+	lockdep_set_class(&lock->wait_lock, &pi_futex_key);
 	rt_mutex_set_owner(lock, proxy_owner);
 }