From dfd718a2ed1f678e66749ffe41bdeafedf3f4314 Mon Sep 17 00:00:00 2001
From: "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>
Date: Mon, 1 Oct 2018 16:10:39 +0530
Subject: [PATCH] powerpc/rtas: Fix a potential race between CPU-Offline &
Migration
References: bsc#1111870
Patch-mainline: v4.20-rc1
Git-commit: dfd718a2ed1f678e66749ffe41bdeafedf3f4314
Live Partition Migrations require all the present CPUs to execute the
H_JOIN call, and hence rtas_ibm_suspend_me() onlines any offline CPUs
before initiating the migration for this purpose.
The commit 85a88cabad57
("powerpc/pseries: Disable CPU hotplug across migrations")
disables any CPU-hotplug operations once all the offline CPUs are
brought online to prevent any further state change. Once the
CPU-Hotplug operation is disabled, the code assumes that all the CPUs
are online.
However, there is a minor window in rtas_ibm_suspend_me() between
onlining the offline CPUs and disabling CPU-Hotplug when a concurrent
CPU-offline operations initiated by the userspace can succeed thereby
nullifying the the aformentioned assumption. In this unlikely case
these offlined CPUs will not call H_JOIN, resulting in a system hang.
Fix this by verifying that all the present CPUs are actually online
after CPU-Hotplug has been disabled, failing which we restore the
state of the offline CPUs in rtas_ibm_suspend_me() and return an
-EBUSY.
Cc: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Cc: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Reviewed-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Michal Suchanek <msuchanek@suse.de>
---
arch/powerpc/kernel/rtas.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
index e02ac377ed64..de35bd8f047f 100644
--- a/arch/powerpc/kernel/rtas.c
+++ b/arch/powerpc/kernel/rtas.c
@@ -983,6 +983,14 @@ int rtas_ibm_suspend_me(u64 handle)
cpu_hotplug_disable();
+ /* Check if we raced with a CPU-Offline Operation */
+ if (unlikely(!cpumask_equal(cpu_present_mask, cpu_online_mask))) {
+ pr_err("%s: Raced against a concurrent CPU-Offline\n",
+ __func__);
+ atomic_set(&data.error, -EBUSY);
+ goto out_hotplug_enable;
+ }
+
/* Call function on all CPUs. One of us will make the
* rtas call
*/
@@ -994,6 +1002,7 @@ int rtas_ibm_suspend_me(u64 handle)
if (atomic_read(&data.error) != 0)
printk(KERN_ERR "Error doing global join\n");
+out_hotplug_enable:
cpu_hotplug_enable();
/* Take down CPUs not online prior to suspend */
--
2.13.7