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Subject[PATCH 4.18 018/197] hv_netvsc: Fix a deadlock by getting rtnl lock earlier in netvsc_probe()
Date
4.18-stable review patch.  If anyone has any objections, please let me know.

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

From: Dexuan Cui <decui@microsoft.com>

[ Upstream commit e04e7a7bbd4bbabef4e1a58367e5fc9b2edc3b10 ]

This patch fixes the race between netvsc_probe() and
rndis_set_subchannel(), which can cause a deadlock.

These are the related 3 paths which show the deadlock:

path #1:
Workqueue: hv_vmbus_con vmbus_onmessage_work [hv_vmbus]
Call Trace:
schedule
schedule_preempt_disabled
__mutex_lock
__device_attach
bus_probe_device
device_add
vmbus_device_register
vmbus_onoffer
vmbus_onmessage_work
process_one_work
worker_thread
kthread
ret_from_fork

path #2:
schedule
schedule_preempt_disabled
__mutex_lock
netvsc_probe
vmbus_probe
really_probe
__driver_attach
bus_for_each_dev
driver_attach_async
async_run_entry_fn
process_one_work
worker_thread
kthread
ret_from_fork

path #3:
Workqueue: events netvsc_subchan_work [hv_netvsc]
Call Trace:
schedule
rndis_set_subchannel
netvsc_subchan_work
process_one_work
worker_thread
kthread
ret_from_fork

Before path #1 finishes, path #2 can start to run, because just before
the "bus_probe_device(dev);" in device_add() in path #1, there is a line
"object_uevent(&dev->kobj, KOBJ_ADD);", so systemd-udevd can
immediately try to load hv_netvsc and hence path #2 can start to run.

Next, path #2 offloads the subchannal's initialization to a workqueue,
i.e. path #3, so we can end up in a deadlock situation like this:

Path #2 gets the device lock, and is trying to get the rtnl lock;
Path #3 gets the rtnl lock and is waiting for all the subchannel messages
to be processed;
Path #1 is trying to get the device lock, but since #2 is not releasing
the device lock, path #1 has to sleep; since the VMBus messages are
processed one by one, this means the sub-channel messages can't be
procedded, so #3 has to sleep with the rtnl lock held, and finally #2
has to sleep... Now all the 3 paths are sleeping and we hit the deadlock.

With the patch, we can make sure #2 gets both the device lock and the
rtnl lock together, gets its job done, and releases the locks, so #1
and #3 will not be blocked for ever.

Fixes: 8195b1396ec8 ("hv_netvsc: fix deadlock on hotplug")
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/net/hyperv/netvsc_drv.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)

--- a/drivers/net/hyperv/netvsc_drv.c
+++ b/drivers/net/hyperv/netvsc_drv.c
@@ -2101,6 +2101,16 @@ static int netvsc_probe(struct hv_device

memcpy(net->dev_addr, device_info.mac_adr, ETH_ALEN);

+ /* We must get rtnl lock before scheduling nvdev->subchan_work,
+ * otherwise netvsc_subchan_work() can get rtnl lock first and wait
+ * all subchannels to show up, but that may not happen because
+ * netvsc_probe() can't get rtnl lock and as a result vmbus_onoffer()
+ * -> ... -> device_add() -> ... -> __device_attach() can't get
+ * the device lock, so all the subchannels can't be processed --
+ * finally netvsc_subchan_work() hangs for ever.
+ */
+ rtnl_lock();
+
if (nvdev->num_chn > 1)
schedule_work(&nvdev->subchan_work);

@@ -2119,7 +2129,6 @@ static int netvsc_probe(struct hv_device
else
net->max_mtu = ETH_DATA_LEN;

- rtnl_lock();
ret = register_netdevice(net);
if (ret != 0) {
pr_err("Unable to register netdev.\n");

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