linux-kernel - Re: bluetooth: fix deadlock on device reset and power down

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Date:	Mon, 2 Apr 2012 10:52:16 -0300
From:	Andre Guedes <andre.guedes@...nbossa.org>
To:	Alexander Holler <holler@...oftware.de>
Cc:	Andrei Emeltchenko <andrei.emeltchenko.news@...il.com>,
	David Herrmann <dh.herrmann@...glemail.com>,
	linux-bluetooth@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Gustavo F. Padovan" <padovan@...fusion.mobi>,
	stable@...r.kernel.org
Subject: Re: bluetooth: fix deadlock on device reset and power down

Hi all,

On Mon, Apr 2, 2012 at 7:16 AM, Alexander Holler <holler@...oftware.de> wrote:
> Am 02.04.2012 11:17, schrieb Alexander Holler:
>
>> 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@...oftware.de>
>>>>  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@...nbossa.org>
>>> 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.
>
>
> Oh, sorry, that patch is not from you, your first name is only similiar.
> I've added the author to cc, just in case the lock might still be needed in
> 3.3 if no other patches (from 3.4) besides that one are applied.

It is still applicable. It was applied to bluetooth-next tree a week
after 3.3 merge window was closed. This is the reason why it is not
present in 3.3.

BR,

Andre
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