[azure 4.13][PATCH 16/19] vmbus: hvsock: add proper sync for vmbus_hvsock_device_unregister()

[Marcelo Henrique Cerri]

From: Dexuan Cui <decui at microsoft.com>

BugLink: http://bugs.launchpad.net/bugs/1736283

Without the patch, vmbus_hvsock_device_unregister() can destroy the device
prematurely when close() is called, and can cause NULl dereferencing or
potential data loss (the last portion of the data stream may be dropped
prematurely).

Signed-off-by: Dexuan Cui <decui at microsoft.com>
Cc: Haiyang Zhang <haiyangz at microsoft.com>
Cc: Stephen Hemminger <sthemmin at microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys at microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
(cherry picked from commit 9d35593b4f0b89ab0c194349c7d357b3b159e99a)
Signed-off-by: Dexuan Cui <decui at microsoft.com>
Signed-off-by: Marcelo Henrique Cerri <marcelo.cerri at canonical.com>
---
 drivers/hv/channel_mgmt.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
index fd5faa37d3cf..86d87ea41ba5 100644
--- a/drivers/hv/channel_mgmt.c
+++ b/drivers/hv/channel_mgmt.c
@@ -949,7 +949,10 @@ void vmbus_hvsock_device_unregister(struct vmbus_channel *channel)
 {
 	BUG_ON(!is_hvsock_channel(channel));
 
-	channel->rescind = true;
+	/* We always get a rescind msg when a connection is closed. */
+	while (!READ_ONCE(channel->probe_done) || !READ_ONCE(channel->rescind))
+		msleep(1);
+
 	vmbus_device_unregister(channel->device_obj);
 }
 EXPORT_SYMBOL_GPL(vmbus_hvsock_device_unregister);
-- 
2.7.4