Re: [PATCH v1 0/7] Avoid L2CAP ERTM shutdown hung tasks

[Dean Jenkins <Dean_Jenkins@mentor.com>]

Hi Luiz,

On 07/10/15 15:09, Luiz Augusto von Dentz wrote:
> Hi Dean,
>
> On Sat, Jun 6, 2015 at 7:13 AM, Marcel Holtmann <marcel@holtmann.org> wrote:
>> Hi Dean,
>>
>>> Details of the issue are described in the thread
>>> [RFC] Bluetooth ERTM L2CAP deadlocks (hung tasks) due to l2cap_sock_shutdown()
>>> and kernel.org Bugzilla https://bugzilla.kernel.org/show_bug.cgi?id=99301
> Are you still working on this? I can still lock bluetoothd on close,
> btw it seems you are not aware of the following:
>
>         SO_LINGER
>                When enabled, a close(2) or shutdown(2) will not return
> until all queued messages for the socket have been successfully sent
> or the linger timeout  has  been  reached.   Otherwise,  the  call
> returns immediately and the closing is done in the background.
>
> So according to the text above we should never call __l2cap_wait_ack
> on l2cap_sock_shutdown since it should return immediately, only when
> SO_LINGER is used it is allowed to block. Also there doesn't seems to
> be anything prevent us to send a L2CAP Disconnect request while there
> are unacked packets, at least I could find anything on the core about
> waiting for unacked nor the test spec has any test for it.
>
Thanks for the information about SO_LINGER time. So it sounds like 
further changes to the logic of the code are needed.

Yes, we are still working on this issue. We have devised a fix for our 
commercial ARM kernel 3.14. and so far we have no reported failures.

In our fix, we had to ensure "L2CAP Disconnect request" took precedence 
over shutdown.

Note that ACKS are only used for L2CAP ERTM and so only some BT services 
risk getting hung such as audio media browsing over BT.

We are currently preparing a patchset for bluetooth-next of our 2nd 
attempt at resolving the locking issue.

Thanks,

Regards,
Dean

-- 
Dean Jenkins
Embedded Software Engineer
Linux Transportation Solutions
Mentor Embedded Software Division
Mentor Graphics (UK) Ltd.