linux-kernel - Re: [PATCH 2/2] hso: fix deadlock when receiving bursts of data

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Date:	Fri, 11 Jul 2014 14:05:42 +0200
From:	Olivier Sobrie <olivier@...rie.be>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	David Miller <davem@...emloft.net>,
	"j.dumon@...ion.com" <j.dumon@...ion.com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] hso: fix deadlock when receiving bursts of data

On Fri, Jul 11, 2014 at 09:28:47AM +0000, David Laight wrote:
> From: Olivier Sobrie Olivier Sobrie
> > Hi Alan and Davids,
> > 
> > On Thu, Jul 10, 2014 at 04:50:03PM +0100, One Thousand Gnomes wrote:
> > > On Thu, 10 Jul 2014 14:37:37 +0000
> > > David Laight <David.Laight@...LAB.COM> wrote:
> > >
> > > > From: Olivier Sobrie
> > > > ...
> > > > > The function put_rxbuf_data() is called from the urb completion handler.
> > > > > It puts the data of the urb transfer in the tty buffer with
> > > > > tty_insert_flip_string_flags() and schedules a work queue in order to
> > > > > push the data to the ldisc.
> > > > > Problem is that we are in a urb completion handler so we can't wait
> > > > > until there is room in the tty buffer.
> > >
> > > The tty provides the input queue, if the queue is full then just chuck
> > > the data in the bitbucket.  hso is trying to be far too clever.
> > >
> > > If hso is fast enough that the buffering isn't sufficient on the tty side
> > > then we need to fix the tty buffer size.
> > 
> > Ok I'll adapt the patch to drop the data that can't be put in the tty
> > buffer. I test this and resend a new patch.
> 
> If you are going to drop data, then ideally you want to discard entire ppp
> packets. Depending on exactly how the interface works it might be that
> urb are likely to contain complete packets.

Indeed, urbs contains complete ppp packets. I see that the first and last
byte are equal to 0x7e.

> 
> So discarding entire urb might work better than discarding a few bytes.
> (But don't even think of scanning the data stream in the usb driver - except
> for experiments.)

I will check with tty_buffer_request_room() that there is enough room to
receive the frame before inserting the data in the tty buffer with
tty_insert_flip_string().

-- 
Olivier
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