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Subject[PATCH 4.9 16/44] s390/smp: fix CPU hotplug deadlock with CPU rescan
Date
4.9-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
@@ -1139,7 +1139,11 @@ 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
@@ -59,7 +59,9 @@ 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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