Re: bluetooth: fix deadlock on device reset and power down

From: Alexander Holler
Date: Mon Apr 02 2012 - 05:17:18 EST


Am 02.04.2012 11:03, schrieb Andrei Emeltchenko:
> Hi guys,
>
> On Mon, Apr 02, 2012 at 10:44:43AM +0200, David Herrmann wrote:
>> Hi Andrei and Alexander
>>
>> On Mon, Apr 2, 2012 at 10:29 AM, Alexander Holler <holler@xxxxxxxxxxxxx> wrote:
>>> Am 02.04.2012 08:55, schrieb Andrei Emeltchenko:
>>>> Hi Alexander,
>>>>
>>>> On Sat, Mar 31, 2012 at 03:23:38PM +0200, Alexander Holler wrote:
>>>>> I've experienced a deadlock on shutdown using kernel 3.3 and tracked
>>>>> it down. Because I'm not very familiar with the bluetooth stack I'm
>>>>> not sure if the below patch is correct, but it fixed the problem
>>>>> here.
>>>>
>>>> Could you please attach deadlock dump?
>>>>
>>>>>
>>>>> Commit 09fd0de5bd8f8ef3317e5365f92f1a13dcd89aa9 introduced a deadlock:
>>>>>
>>>>> bluetoothd calls ioctl HCIDEVDOWN
>>>>> hci_sock_ioctl()
>>>>> hci_dev_close()
>>>>> hci_dev_do_close()
>>>>> hci_dev_lock(hdev);
>>>>> inquiry_cache_flush();
>>>>> hci_conn_hash_flush();
>>>>> hci_conn_del()
>>>>> cancel_delayed_work_sync()
>>>>> hci_conn_timeout()
>>>>> hci_dev_lock(hdev); /* DEADLOCK */
>>>>
>>>> I am actually not sure that hci_conn_timeout locks hdev. Why do you think
>>>> so?
>>>
>>> By reading the source, printk and suffering through the deadlock. It's
>>> especially painfull when using a bt-keyboard and systemd, because
>>> systemd tries 4 times (~ some minutes) to kill bluetoothd before it
>>> marks the service as failed and finally continues to shut down.
>>
>> hci_conn_timeout does lock the device. See the source. But the problem
>
> I think you need to check commit e72acc13c770a82b4ce4a07e9716f29320eae0f8
>
> commit e72acc13c770a82b4ce4a07e9716f29320eae0f8
> Author: Andre Guedes <andre.guedes@xxxxxxxxxxxxx>
> Date: Fri Jan 27 19:42:03 2012 -0300
>
> Bluetooth: Remove unneeded locking
>
> We don't need locking hdev in hci_conn_timeout() since it doesn't
> access any hdev's shared resources, it basically queues HCI commands.

So if the locks in hci_conn_timeout() aren't needed, your commit which
removes them should go to the stable tree because it fixes a painful
deadlock.

Regards,

Alexander

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