[Xen-devel] Xen Linux deadlock

[Andre Przywara <andre.przywara@xxxxxxx>]

Hi,

when booting Linux 4.12-rc4 as Dom0 under a recent Xen HV I saw the
following lockdep splat after running xencommons start:

root@junor1:~# bash /etc/init.d/xencommons start
Setting domain 0 name, domid and JSON config...
[  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
[  248.145214]
[  248.145214] other info that might help us debug this:
[  248.145214]
[  248.153383]  Possible unsafe locking scenario:
[  248.153383]
[  248.159403]        CPU0                    CPU1
[  248.163960]        ----                    ----
[  248.168518]   lock(xb_write_mutex);
[  248.172045]                                lock(&u->msgbuffer_mutex);
[  248.178500]                                lock(xb_write_mutex);
[  248.184514]   lock(&u->msgbuffer_mutex);
[  248.188470]
[  248.188470]  *** DEADLOCK ***
[  248.188470]
[  248.194578] 2 locks held by xenbus/91:
[  248.198360]  #0:  (xs_response_mutex){+.+...}, at:
[<ffff00000863a7b0>] xenbus_thread+0x460/0x798
[  248.207218]  #1:  (xb_write_mutex){+.+...}, at: [<ffff00000863a940>]
xenbus_thread+0x5f0/0x798
[  248.215818]
[  248.215818] stack backtrace:
[  248.220293] CPU: 0 PID: 91 Comm: xenbus Not tainted
4.12.0-rc4-00022-gc4b25c0 #575
[  248.227858] Hardware name: ARM Juno development board (r1) (DT)
[  248.233792] Call trace:
[  248.236289] [<ffff00000808a748>] dump_backtrace+0x0/0x270
[  248.241707] [<ffff00000808aa94>] show_stack+0x24/0x30
[  248.246782] [<ffff0000084caa98>] dump_stack+0xb8/0xf0
[  248.251859] [<ffff000008139068>] print_circular_bug+0x1f8/0x2e0
[  248.257787] [<ffff00000813c090>] __lock_acquire+0x1768/0x1778
[  248.263548] [<ffff00000813c90c>] lock_acquire+0xc4/0x288
[  248.268882] [<ffff000008bdb28c>] __mutex_lock+0x84/0x868
[  248.274219] [<ffff000008bdbaac>] mutex_lock_nested+0x3c/0x50
[  248.279889] [<ffff00000863e904>] xenbus_dev_queue_reply+0x3c/0x230
[  248.286081] [<ffff00000863aad8>] xenbus_thread+0x788/0x798
[  248.291585] [<ffff000008108070>] kthread+0x110/0x140
[  248.296572] [<ffff000008083710>] ret_from_fork+0x10/0x40

Apparently it's not easily reproducible, but Julien confirmed that the
dead lock condition as reported above is indeed in the Linux code.

Does anyone has an idea of how to fix this?

Cheers,
Andre.

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