From b7cb707c373094ce4008d4a6ac9b6b366ec52da5 Mon Sep 17 00:00:00 2001 From: Gerald Schaefer Date: Wed, 9 Jan 2019 13:00:03 +0100 Subject: s390/smp: fix CPU hotplug deadlock with CPU rescan 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: Signed-off-by: Gerald Schaefer Signed-off-by: Martin Schwidefsky --- arch/s390/kernel/smp.c | 4 ++++ 1 file changed, 4 insertions(+) (limited to 'arch/s390') diff --git a/arch/s390/kernel/smp.c b/arch/s390/kernel/smp.c index f82b3d3c36e2..307a1c86ea21 100644 --- a/arch/s390/kernel/smp.c +++ b/arch/s390/kernel/smp.c @@ -1166,7 +1166,11 @@ static ssize_t __ref rescan_store(struct device *dev, { int rc; + rc = lock_device_hotplug_sysfs(); + if (rc) + return rc; rc = smp_rescan_cpus(); + unlock_device_hotplug(); return rc ? rc : count; } static DEVICE_ATTR_WO(rescan); -- cgit v1.2.3