linux-kernel - [tip: sched/urgent] membarrier: Fix RCU locking bug caused by faulty merge

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Date:   Tue, 01 Oct 2019 19:34:11 -0000
From:   "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Christoph Lameter <cl@...ux.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kirill Tkhai <tkhai@...dex.ru>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Mike Galbraith <efault@....de>,
        Oleg Nesterov <oleg@...hat.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        "Russell King - ARM Linux admin" <linux@...linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-tip-commits@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [tip: sched/urgent] membarrier: Fix RCU locking bug caused by faulty merge

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     73956fc07dd7b25d4a33ab3fdd6247c60d0b237c
Gitweb:        https://git.kernel.org/tip/73956fc07dd7b25d4a33ab3fdd6247c60d0b237c
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Tue, 01 Oct 2019 10:50:33 +02:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Tue, 01 Oct 2019 21:27:50 +02:00

membarrier: Fix RCU locking bug caused by faulty merge

The following commit:

  227a4aadc75b ("sched/membarrier: Fix p->mm->membarrier_state racy load")

got fat fingered by me when merging it with other patches. It meant to move
the RCU section out of the for loop but ended up doing it partially, leaving
a superfluous rcu_read_lock() inside, causing havok.

Reported-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Chris Metcalf <cmetcalf@...hip.com>
Cc: Christoph Lameter <cl@...ux.com>
Cc: Eric W. Biederman <ebiederm@...ssion.com>
Cc: Kirill Tkhai <tkhai@...dex.ru>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Mike Galbraith <efault@....de>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Paul E. McKenney <paulmck@...ux.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-tip-commits@...r.kernel.org
Fixes: 227a4aadc75b ("sched/membarrier: Fix p->mm->membarrier_state racy load")
Link: https://lkml.kernel.org/r/20191001085033.GP4519@hirez.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/membarrier.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index a39bed2..168479a 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -174,7 +174,6 @@ static int membarrier_private_expedited(int flags)
 		 */
 		if (cpu == raw_smp_processor_id())
 			continue;
-		rcu_read_lock();
 		p = rcu_dereference(cpu_rq(cpu)->curr);
 		if (p && p->mm == mm)
 			__cpumask_set_cpu(cpu, tmpmask);