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    Subject[PATCH 4.20 047/117] s390/smp: fix CPU hotplug deadlock with CPU rescan
    Date
    4.20-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    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)

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