[PATCH 4.20 047/117] s390/smp: fix CPU hotplug deadlock with CPU rescan

[Greg Kroah-Hartman]

4.20-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Gerald Schaefer <gerald.schaefer@xxxxxxxxxx>

commit b7cb707c373094ce4008d4a6ac9b6b366ec52da5 upstream.

smp_rescan_cpus() is called without the device_hotplug_lock, which can lead
to a dedlock when a new CPU is found and immediately set online by a udev
rule.

This was observed on an older kernel version, where the cpu_hotplug_begin()
loop was still present, and it resulted in hanging chcpu and systemd-udev
processes. This specific deadlock will not show on current kernels. However,
there may be other possible deadlocks, and since smp_rescan_cpus() can still
trigger a CPU hotplug operation, the device_hotplug_lock should be held.

For reference, this was the deadlock with the old cpu_hotplug_begin() loop:

        chcpu (rescan)                       systemd-udevd

 echo 1 > /sys/../rescan
 -> smp_rescan_cpus()
 -> (*) get_online_cpus()
    (increases refcount)
 -> smp_add_present_cpu()
    (new CPU found)
 -> register_cpu()
 -> device_add()
 -> udev "add" event triggered -----------> udev rule sets CPU online
                                         -> echo 1 > /sys/.../online
                                         -> lock_device_hotplug_sysfs()
                                            (this is missing in rescan path)
                                         -> device_online()
                                         -> (**) device_lock(new CPU dev)
                                         -> cpu_up()
                                         -> cpu_hotplug_begin()
                                            (loops until refcount == 0)
                                            -> deadlock with (*)
 -> bus_probe_device()
 -> device_attach()
 -> device_lock(new CPU dev)
    -> deadlock with (**)

Fix this by taking the device_hotplug_lock in the CPU rescan path.

Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxx>
Signed-off-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/s390/kernel/smp.c          |    4 ++++
 drivers/s390/char/sclp_config.c |    2 ++
 2 files changed, 6 insertions(+)

--- a/arch/s390/kernel/smp.c
+++ b/arch/s390/kernel/smp.c
@@ -1166,7 +1166,11 @@ static ssize_t __ref rescan_store(struct
 {
 	int rc;
 
+	rc = lock_device_hotplug_sysfs();
+	if (rc)
+		return rc;
 	rc = smp_rescan_cpus();
+	unlock_device_hotplug();
 	return rc ? rc : count;
 }
 static DEVICE_ATTR_WO(rescan);
--- a/drivers/s390/char/sclp_config.c
+++ b/drivers/s390/char/sclp_config.c
@@ -60,7 +60,9 @@ static void sclp_cpu_capability_notify(s
 
 static void __ref sclp_cpu_change_notify(struct work_struct *work)
 {
+	lock_device_hotplug();
 	smp_rescan_cpus();
+	unlock_device_hotplug();
 }
 
 static void sclp_conf_receiver_fn(struct evbuf_header *evbuf)