original_kernel/drivers/s390/char
Gerald Schaefer b7cb707c37 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: <stable@vger.kernel.org>
Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
2019-01-11 17:12:02 +01:00
..
Kconfig
Makefile
con3215.c
con3270.c
ctrlchar.c
ctrlchar.h
defkeymap.c
defkeymap.map
diag_ftp.c
diag_ftp.h
fs3270.c
hmcdrv_cache.c
hmcdrv_cache.h
hmcdrv_dev.c
hmcdrv_dev.h
hmcdrv_ftp.c
hmcdrv_ftp.h
hmcdrv_mod.c
keyboard.c
keyboard.h
monreader.c
monwriter.c
raw3270.c
raw3270.h
sclp.c
sclp.h
sclp_async.c
sclp_cmd.c
sclp_con.c
sclp_config.c
sclp_cpi_sys.c
sclp_cpi_sys.h
sclp_ctl.c
sclp_diag.h
sclp_early.c
sclp_early_core.c
sclp_ftp.c
sclp_ftp.h
sclp_ocf.c
sclp_pci.c
sclp_quiesce.c
sclp_rw.c
sclp_rw.h
sclp_sd.c
sclp_sdias.c
sclp_sdias.h
sclp_tty.c
sclp_tty.h
sclp_vt220.c
tape.h
tape_34xx.c
tape_3590.c
tape_3590.h
tape_char.c
tape_class.c
tape_class.h
tape_core.c
tape_proc.c
tape_std.c
tape_std.h
tty3270.c
tty3270.h
vmcp.c
vmlogrdr.c
vmur.c
vmur.h
zcore.c