Re: [REGRESSION] "USB: use kfifo to buffer usb-generic serial writes" causes gobi_loader to hang

From: Oliver Neukum <oliver@neukum.org>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Johan Hovold <jhovold@gmail.com>,
	Matthew Garrett <mjg59@srcf.ucam.org>,
	dvomlehn@cisco.com, gregkh@suse.de, linux-usb@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [REGRESSION] "USB: use kfifo to buffer usb-generic serial writes" causes gobi_loader to hang
Date: Tue, 19 Jan 2010 19:44:54 +0100	[thread overview]
Message-ID: <201001191944.54622.oliver@neukum.org> (raw)
In-Reply-To: <20100119162746.04ba6d9e@lxorguk.ukuu.org.uk>

Am Dienstag, 19. Januar 2010 17:27:46 schrieb Alan Cox:
> On Tue, 19 Jan 2010 16:25:36 +0100
> Johan Hovold <jhovold@gmail.com> wrote:

> > Is it really a fix? If the fifo is already full the write urb should be
> > in use and Oliver's patch would amount to only a minor optimisation as
> > usb_serial_generic_write_start would return 0 anyway.
> 
> IF the write returns a zero then it will sleep in n_tty waiting for a
> wakeup when the FIFO level drops sufficiently. If that isn't working
> check that all cases where data is cleared from the FIFO called
> tty_wakeup and do so *after* the FIFO has been partly emptied and the
> locking has ensured the space is visible to the write side.
> 
usb_serial_generic_write_bulk_callback() always calls the softint
in the single URB case. Therefore the test with the patch I sent.
It is unlikely to be chance that the hang happens just as the FIFO
is full.

	Regards
		Oliver