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linux/drivers/s390
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Gerald Schaefer 049c7b068d s390/smp: fix CPU hotplug deadlock with CPU rescan 
commit b7cb707c37 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>
2019-01-31 08:14:35 +01:00
..
block libnvdimm-for-4.19_misc 2018-08-25 18:13:10 -07:00
char s390/smp: fix CPU hotplug deadlock with CPU rescan 2019-01-31 08:14:35 +01:00
cio s390/cio: Fix cleanup when unsupported IDA format is used 2018-12-17 09:24:31 +01:00
crypto s390/zcrypt: remove VLA usage from the AP bus 2018-09-12 11:33:37 +02:00
net s390/ism: clear dmbe_mask bit before SMC IRQ handling 2018-12-13 09:16:11 +01:00
scsi scsi: zfcp: fix posting too many status read buffers leading to adapter shutdown 2019-01-13 09:51:03 +01:00
virtio virtio/s390: fix race in ccw_io_helper() 2018-12-13 09:16:18 +01:00
Makefile kbuild: rename built-in.o to built-in.a 2018-03-26 02:01:19 +09:00