• Gerald Schaefer's avatar
    s390/smp: fix CPU hotplug deadlock with CPU rescan · b7cb707c
    Gerald Schaefer authored
    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: default avatarGerald Schaefer <gerald.schaefer@de.ibm.com>
    Signed-off-by: default avatarMartin Schwidefsky <schwidefsky@de.ibm.com>
    b7cb707c
sclp_config.c 4.06 KB