From: Peter Zijlstra <peterz@infradead.org>
Date: Tue, 1 Oct 2019 08:50:33 +0000 (+0200)
Subject: membarrier: Fix RCU locking bug caused by faulty merge
X-Git-Tag: Ubuntu-5.4-5.4.0-11.14~1907^2
X-Git-Url: https://git.proxmox.com/?a=commitdiff_plain;h=73956fc07dd7b25d4a33ab3fdd6247c60d0b237c;p=mirror_ubuntu-focal-kernel.git

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@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Kirill Tkhai <tkhai@yandex.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-tip-commits@vger.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@kernel.org>
---

diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index a39bed2c784f..168479a7d61b 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);