[3.16.y-ckt stable] Patch "powerpc/smp: Wait until secondaries are active & online" has been added to staging queue

Luis Henriques luis.henriques at canonical.com
Mon Mar 30 13:01:58 UTC 2015

This is a note to let you know that I have just added a patch titled

    powerpc/smp: Wait until secondaries are active & online

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:


This patch is scheduled to be released in version 3.16.7-ckt10.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see



>From 2b86feb696b4c3ffa2ad60e9e154b168ec2acdd3 Mon Sep 17 00:00:00 2001
From: Michael Ellerman <mpe at ellerman.id.au>
Date: Tue, 24 Feb 2015 17:58:02 +1100
Subject: powerpc/smp: Wait until secondaries are active & online

commit 875ebe940d77a41682c367ad799b4f39f128d3fa upstream.

Anton has a busy ppc64le KVM box where guests sometimes hit the infamous
"kernel BUG at kernel/smpboot.c:134!" issue during boot:

  BUG_ON(td->cpu != smp_processor_id());

Basically a per CPU hotplug thread scheduled on the wrong CPU. The oops
output confirms it:

  CPU: 0
  Comm: watchdog/130

The problem is that we aren't ensuring the CPU active bit is set for the
secondary before allowing the master to continue on. The master unparks
the secondary CPU's kthreads and the scheduler looks for a CPU to run
on. It calls select_task_rq() and realises the suggested CPU is not in
the cpus_allowed mask. It then ends up in select_fallback_rq(), and
since the active bit isnt't set we choose some other CPU to run on.

This seems to have been introduced by 6acbfb96976f "sched: Fix hotplug
vs. set_cpus_allowed_ptr()", which changed from setting active before
online to setting active after online. However that was in turn fixing a
bug where other code assumed an active CPU was also online, so we can't
just revert that fix.

The simplest fix is just to spin waiting for both active & online to be
set. We already have a barrier prior to set_cpu_online() (which also
sets active), to ensure all other setup is completed before online &
active are set.

Fixes: 6acbfb96976f ("sched: Fix hotplug vs. set_cpus_allowed_ptr()")
Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
Signed-off-by: Anton Blanchard <anton at samba.org>
Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
 arch/powerpc/kernel/smp.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 1007fb802e6b..383859d125de 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -546,8 +546,8 @@ int __cpu_up(unsigned int cpu, struct task_struct *tidle)
 	if (smp_ops->give_timebase)

-	/* Wait until cpu puts itself in the online map */
-	while (!cpu_online(cpu))
+	/* Wait until cpu puts itself in the online & active maps */
+	while (!cpu_online(cpu) || !cpu_active(cpu))

 	return 0;

More information about the kernel-team mailing list