[PATCH Lucid SRU] 9p: Fix the kernel crash on a failed mount

[Tim Gardner]

From: "Aneesh Kumar K.V" <aneesh.kumar at linux.vnet.ibm.com>

BugLink: http://bugs.launchpad.net/bugs/676823

The patch fix the crash repoted below

[   15.149907] BUG: unable to handle kernel NULL pointer dereference at 00000001
[   15.150806] IP: [<c140b886>] p9_virtio_close+0x18/0x24
.....
....
[   15.150806] Call Trace:
[   15.150806]  [<c1408e78>] ? p9_client_destroy+0x3f/0x163
[   15.150806]  [<c1409342>] ? p9_client_create+0x25f/0x270
[   15.150806]  [<c1063b72>] ? trace_hardirqs_on+0xb/0xd
[   15.150806]  [<c11ed4e8>] ? match_token+0x64/0x164
[   15.150806]  [<c1175e8d>] ? v9fs_session_init+0x2f1/0x3c8
[   15.150806]  [<c109cfc9>] ? kmem_cache_alloc+0x98/0xb8
[   15.150806]  [<c1063b72>] ? trace_hardirqs_on+0xb/0xd
[   15.150806]  [<c1173dd1>] ? v9fs_get_sb+0x47/0x1e8
[   15.150806]  [<c1173dea>] ? v9fs_get_sb+0x60/0x1e8
[   15.150806]  [<c10a2e77>] ? vfs_kern_mount+0x81/0x11a
[   15.150806]  [<c10a2f55>] ? do_kern_mount+0x33/0xbe
[   15.150806]  [<c10b40b9>] ? do_mount+0x654/0x6b3
[   15.150806]  [<c1038949>] ? do_page_fault+0x0/0x284
[   15.150806]  [<c10b28ec>] ? copy_mount_options+0x73/0xd2
[   15.150806]  [<c10b4179>] ? sys_mount+0x61/0x94
[   15.150806]  [<c14284e9>] ? syscall_call+0x7/0xb
....
[   15.203562] ---[ end trace 1dd159357709eb4b ]---
[

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.vnet.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh at gmail.com>
(cherry picked from commit fb786100f7c75e154e63d0f5a2982e6d46dfb602)

Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
---
 net/9p/trans_virtio.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
index 67c4bc7..cb50f4a 100644
--- a/net/9p/trans_virtio.c
+++ b/net/9p/trans_virtio.c
@@ -102,7 +102,8 @@ static void p9_virtio_close(struct p9_client *client)
 	struct virtio_chan *chan = client->trans;
 
 	mutex_lock(&virtio_9p_lock);
-	chan->inuse = false;
+	if (chan)
+		chan->inuse = false;
 	mutex_unlock(&virtio_9p_lock);
 }
 
-- 
1.7.9.5