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

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Date:	Mon, 02 Apr 2012 11:17:12 +0200
From:	Alexander Holler <holler@...oftware.de>
To:	Andrei Emeltchenko <andrei.emeltchenko.news@...il.com>
CC:	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

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.

Regards,

Alexander

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