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Date: Mon, 4 Feb 2019 11:36:05 +0100 From: Greg Kroah-Hartman <gregkh@...uxfoundation.org> To: linux-kernel@...r.kernel.org Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, stable@...r.kernel.org, Gerald Schaefer <gerald.schaefer@...ibm.com>, Martin Schwidefsky <schwidefsky@...ibm.com> Subject: [PATCH 4.4 12/65] s390/smp: fix CPU hotplug deadlock with CPU rescan 4.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Gerald Schaefer <gerald.schaefer@...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@...r.kernel.org> Signed-off-by: Gerald Schaefer <gerald.schaefer@...ibm.com> Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.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 @@ -1152,7 +1152,11 @@ 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(rescan, 0200, NULL, rescan_store); --- a/drivers/s390/char/sclp_config.c +++ b/drivers/s390/char/sclp_config.c @@ -43,7 +43,9 @@ 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)
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