linux-kernel - [tip: locking/core] locking/rtmutex: Prevent lockdep false positive with PI futexes

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Date:   Tue, 17 Aug 2021 20:13:59 -0000
From:   "tip-bot2 for Thomas Gleixner" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: locking/core] locking/rtmutex: Prevent lockdep false positive
 with PI futexes

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     51711e825a6d1b2fe7ca46bb06d08c25d97656ee
Gitweb:        https://git.kernel.org/tip/51711e825a6d1b2fe7ca46bb06d08c25d97656ee
Author:        Thomas Gleixner <tglx@...utronix.de>
AuthorDate:    Sun, 15 Aug 2021 23:29:20 +02:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Tue, 17 Aug 2021 19:06:02 +02:00

locking/rtmutex: Prevent lockdep false positive with PI futexes

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 separate key.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Link: https://lore.kernel.org/r/20210815211305.750701219@linutronix.de
---
 kernel/locking/rtmutex_api.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/locking/rtmutex_api.c b/kernel/locking/rtmutex_api.c
index 92b7d28..5c9299a 100644
--- a/kernel/locking/rtmutex_api.c
+++ b/kernel/locking/rtmutex_api.c
@@ -214,7 +214,19 @@ EXPORT_SYMBOL_GPL(__rt_mutex_init);
 void __sched rt_mutex_init_proxy_locked(struct rt_mutex_base *lock,
 					struct task_struct *proxy_owner)
 {
+	static struct lock_class_key pi_futex_key;
+
 	__rt_mutex_base_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 separate key.
+	 */
+	lockdep_set_class(&lock->wait_lock, &pi_futex_key);
 	rt_mutex_set_owner(lock, proxy_owner);
 }