linux-kernel - Re: Linux 2.6.28-rc2 i/o error on /dev/ttyUSB0

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Message-ID: <20081030172251.2298b404@lxorguk.ukuu.org.uk>
Date:	Thu, 30 Oct 2008 17:22:51 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Helge Hafting <helge.hafting@...el.hist.no>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.28-rc2 i/o error on /dev/ttyUSB0

> It looks partially fixed.
> No more I/O errors, and the gps unit works fine.
> 
> But look at the attached dmesg file - there are WARNINGs still.

Thats an unrelated and long long standing bug - the USB serial close paths
are all racy versus the received URB handlers. The only reason you now
will occasionally get a warning message is that we bother to actually trap
the case rather than praying silently it doesn't blow up.

What actually occurs is that you enter usb-serial:usb_serial_close which
then drops port->port.count and calls type->close. Somewhere in there new
data arrives and the ldisc path gets to run. The n_tty path tries to echo
back bytes to the (closed) port and the WARN triggers.

Changing the order to clear the port->tty first requires auditing each and
every USB serial driver close method so isn't planned for this release -
but its not caused major disasters in the past few years the race has been
there.

I can push patches this release for it if Linus particularly wants
however.

Alan
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