powerpc/rtas: Fix a potential race between CPU-Offline & Migration

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 85a88cab
("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>

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 */