linux-kernel - Re: hci_ldsic nested locking problem

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Message-ID: <532B25FC.3070408@hurleysoftware.com>
Date:	Thu, 20 Mar 2014 13:31:40 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	balbi@...com, Marcel Holtmann <marcel@...tmann.org>
CC:	Alan Cox <alan@...ux.intel.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	Muralidharan Karicheri <m-karicheri2@...com>,
	linux-bluetooth@...r.kernel.org, linux-serial@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Huang Shijie <b32955@...escale.com>
Subject: Re: hci_ldsic nested locking problem

[ +cc Huang Shijie ]

On 03/20/2014 01:16 PM, Felipe Balbi wrote:
> On Thu, Mar 20, 2014 at 04:42:16PM +0000, Alan Cox wrote:
>> On Thu, 2014-03-20 at 11:34 -0500, Felipe Balbi wrote:
>>> Hi,
>>>
>>> when 8250 driver calls uart_write_wakeup(), the tty port lock is already
>>> taken. hci_ldisc.c's implementation of ->write_wakeup() calls
>>> tty->ops->write() to actually send the characters, but that call will
>>> try to acquire the same port lock again.
>>>
>>> Looking at other line disciplines that looks like a bug in hci_ldisc.c.
>>> Am I correct to assume that ->write_wakeup() is supposed to *just*
>>> wakeup the bottom half so we handle ->write() in another context ?
>>>
>>> Is it legal to call tty->ops->write() from within ->write_wakeup() ?
>>
>> It isn't because you might send all the bytes and go
>>
>> 	write
>> 		write_wakeup
>> 			write
>> 				write wakeup
>> 					...
>>
>> and recurse
>
> cool, so there really is a bug in hci_ldisc. Marcel, any tips on how do
> you want this to be sorted out ?

hci_uart_tx_wakeup() should perform the I/O as work.
FWIW, this was reported by Huang Shijie back on Dec 6.

I'd fix it but I have no way to test it.

Regards,
Peter Hurley
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