Commit 86dd006c authored by Gerald Schaefer's avatar Gerald Schaefer Committed by Greg Kroah-Hartman

s390/smp: fix CPU hotplug deadlock with CPU rescan

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

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: <>
Signed-off-by: default avatarGerald Schaefer <>
Signed-off-by: default avatarMartin Schwidefsky <>
Signed-off-by: default avatarGreg Kroah-Hartman <>
parent 74be2fcd
......@@ -1152,7 +1152,11 @@ static ssize_t __ref rescan_store(struct device *dev,
int rc;
rc = lock_device_hotplug_sysfs();
if (rc)
return rc;
rc = smp_rescan_cpus();
return rc ? rc : count;
static DEVICE_ATTR(rescan, 0200, NULL, rescan_store);
......@@ -43,7 +43,9 @@ static void sclp_cpu_capability_notify(struct work_struct *work)
static void __ref sclp_cpu_change_notify(struct work_struct *work)
static void sclp_conf_receiver_fn(struct evbuf_header *evbuf)
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