4.20-stable review patch. If anyone has any objections, please let me know.
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From: Gerald Schaefer gerald.schaefer@de.ibm.com
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@vger.kernel.org Signed-off-by: Gerald Schaefer gerald.schaefer@de.ibm.com Signed-off-by: Martin Schwidefsky schwidefsky@de.ibm.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- 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)