There has been a report about a deadlock in the xenbus driver:

[  247.979498] ======================================================
[  247.985688] WARNING: possible circular locking dependency detected
[  247.991882] 4.12.0-rc4-00022-gc4b25c0 #575 Not tainted
[  247.997040] ------------------------------------------------------
[  248.003232] xenbus/91 is trying to acquire lock:
[  248.007875]  (&u->msgbuffer_mutex){+.+.+.}, at: [<ffff00000863e904>]
xenbus_dev_queue_reply+0x3c/0x230
[  248.017163]
[  248.017163] but task is already holding lock:
[  248.023096]  (xb_write_mutex){+.+...}, at: [<ffff00000863a940>]
xenbus_thread+0x5f0/0x798
[  248.031267]
[  248.031267] which lock already depends on the new lock.
[  248.031267]
[  248.039615]
[  248.039615] the existing dependency chain (in reverse order) is:
[  248.047176]
[  248.047176] -> #1 (xb_write_mutex){+.+...}:
[  248.052943]        __lock_acquire+0x1728/0x1778
[  248.057498]        lock_acquire+0xc4/0x288
[  248.061630]        __mutex_lock+0x84/0x868
[  248.065755]        mutex_lock_nested+0x3c/0x50
[  248.070227]        xs_send+0x164/0x1f8
[  248.074015]        xenbus_dev_request_and_reply+0x6c/0x88
[  248.079427]        xenbus_file_write+0x260/0x420
[  248.084073]        __vfs_write+0x48/0x138
[  248.088113]        vfs_write+0xa8/0x1b8
[  248.091983]        SyS_write+0x54/0xb0
[  248.095768]        el0_svc_naked+0x24/0x28
[  248.099897]
[  248.099897] -> #0 (&u->msgbuffer_mutex){+.+.+.}:
[  248.106088]        print_circular_bug+0x80/0x2e0
[  248.110730]        __lock_acquire+0x1768/0x1778
[  248.115288]        lock_acquire+0xc4/0x288
[  248.119417]        __mutex_lock+0x84/0x868
[  248.123545]        mutex_lock_nested+0x3c/0x50
[  248.128016]        xenbus_dev_queue_reply+0x3c/0x230
[  248.133005]        xenbus_thread+0x788/0x798
[  248.137306]        kthread+0x110/0x140
[  248.141087]        ret_from_fork+0x10/0x40

It is rather easy to avoid by dropping xb_write_mutex before calling
xenbus_dev_queue_reply().

Fixes: fd8aa909 ("xen: optimize xenbus
driver for multiple concurrent xenstore accesses").

Cc: <stable@vger.kernel.org> # 4.11
Reported-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Tested-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Juergen Gross <jgross@suse.com>