linux-kernel - [tip: core/rcu] locking/rtmutex: rcu: Add WRITE_ONCE() to rt

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Date:   Tue, 24 Mar 2020 09:16:53 -0000
From:   "tip-bot2 for Paul E. McKenney" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        x86 <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: [tip: core/rcu] locking/rtmutex: rcu: Add WRITE_ONCE() to rt_mutex ->owner

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

Commit-ID:     0050c7b2d27c3cc126df59bd8094fb3d25b00ade
Gitweb:        https://git.kernel.org/tip/0050c7b2d27c3cc126df59bd8094fb3d25b00ade
Author:        Paul E. McKenney <paulmck@...nel.org>
AuthorDate:    Fri, 03 Jan 2020 15:59:12 -08:00
Committer:     Paul E. McKenney <paulmck@...nel.org>
CommitterDate: Thu, 20 Feb 2020 15:58:22 -08:00

locking/rtmutex: rcu: Add WRITE_ONCE() to rt_mutex ->owner

The rt_mutex structure's ->owner field is read locklessly, so this
commit adds the WRITE_ONCE() to an update in order to provide proper
documentation and READ_ONCE()/WRITE_ONCE() pairing.

This data race was reported by KCSAN.  Not appropriate for backporting
due to failure being unlikely.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Will Deacon <will@...nel.org>
---
 kernel/locking/rtmutex.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 851bbb1..c9f090d 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -57,7 +57,7 @@ rt_mutex_set_owner(struct rt_mutex *lock, struct task_struct *owner)
 	if (rt_mutex_has_waiters(lock))
 		val |= RT_MUTEX_HAS_WAITERS;
 
-	lock->owner = (struct task_struct *)val;
+	WRITE_ONCE(lock->owner, (struct task_struct *)val);
 }
 
 static inline void clear_rt_mutex_waiters(struct rt_mutex *lock)