Hi Jürgen,
On 08/06/17 15:00, Juergen Gross wrote:
> On 07/06/17 18:24, Juergen Gross wrote:
>> 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 fd8aa9095a95c02dcc35540a263267c29b8fda9d ("xen: optimize xenbus
>> driver for multiple concurrent xenstore accesses").
>>
>> Cc: <stable@xxxxxxxxxxxxxxx> # 4.11
>> Reported-by: Andre Przywara <andre.przywara@xxxxxxx>
>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>
> While this patch is functionally okay, the resulting code is not
> very nice. Will send out V2 soon looking much better.
Thanks anyway for the quick reaction! I will try tomorrow if I can
reproduce the old problem and then confirm that the patch fixes it. I
think I saw xencommons fail somehow (wrong xen-tools version or using
/bin/sh), then fixed that, retried and saw the splat.
Cheers,
Andre.
> Juergen
>
>> ---
>> drivers/xen/xenbus/xenbus_comms.c | 7 +++++--
>> 1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/xen/xenbus/xenbus_comms.c b/drivers/xen/xenbus/xenbus_comms.c
>> index 856ada5d39c9..a44bcdbf6533 100644
>> --- a/drivers/xen/xenbus/xenbus_comms.c
>> +++ b/drivers/xen/xenbus/xenbus_comms.c
>> @@ -305,18 +305,21 @@ static int process_msg(void)
>> req->body = state.body;
>> req->state = xb_req_state_got_reply;
>> list_del(&req->list);
>> + mutex_unlock(&xb_write_mutex);
>> req->cb(req);
>> } else {
>> list_del(&req->list);
>> + mutex_unlock(&xb_write_mutex);
>> kfree(req);
>> }
>> err = 0;
>> break;
>> }
>> }
>> - mutex_unlock(&xb_write_mutex);
>> - if (err)
>> + if (err) {
>> + mutex_unlock(&xb_write_mutex);
>> goto out;
>> + }
>> }
>>
>> mutex_unlock(&xs_response_mutex);
>>
>