[S390] sclp: fix possible deadlock on cpu rescan.

smp_rescan_cpus() calls get_online_cpus() from a multithreaded
workqueue context. This may deadlock. This is the same bug as in
arch/s390/kernel/topology.c. This patch can be reverted as soon as
Oleg's patch gets merged.
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/char/sclp_config.c

@@ -8,6 +8,7 @@
 #include <linux/init.h>
 #include <linux/errno.h>
 #include <linux/cpu.h>
+#include <linux/kthread.h>
 #include <linux/sysdev.h>
 #include <linux/workqueue.h>
 #include <asm/smp.h>
@@ -40,9 +41,19 @@ static void sclp_cpu_capability_notify(struct work_struct *work)
 	put_online_cpus();
 }
 
-static void __ref sclp_cpu_change_notify(struct work_struct *work)
+static int sclp_cpu_kthread(void *data)
 {
 	smp_rescan_cpus();
+	return 0;
+}
+
+static void __ref sclp_cpu_change_notify(struct work_struct *work)
+{
+	/* Can't call smp_rescan_cpus() from  workqueue context since it may
+	 * deadlock in case of cpu hotplug. So we have to create a kernel
+	 * thread in order to call it.
+	 */
+	kthread_run(sclp_cpu_kthread, NULL, "cpu_rescan");
 }
 
 static void sclp_conf_receiver_fn(struct evbuf_header *evbuf)