linux-kernel - Re: [PATCH] tty: Prevent deadlock in n

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Message-ID: <20130130051213.GA26675@kroah.com>
Date:	Wed, 30 Jan 2013 00:12:14 -0500
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Dirkjan Bussink <dirkjan.bussink@...ap.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tty: Prevent deadlock in n_gsm driver

On Tue, Jan 29, 2013 at 09:49:52AM +0100, Dirkjan Bussink wrote:
> This change fixes a deadlock when the multiplexer is closed while there
> are still client side ports open.
> 
> When the multiplexer is closed and there are active tty's it tries to
> close them with tty_vhangup. This has a problem though, because
> tty_vhangup needs the tty_lock. This patch changes it to unlock the
> tty_lock before attempting the hangup and relocks afterwards. The
> additional call to tty_port_tty_set is needed because otherwise the
> port stays active because of the reference counter.
> 
> This change also exposed another problem that other code paths don't
> expect that the multiplexer could have been closed. This patch also adds
> checks for these cases in the gsmtty_ class of function that could be
> called.
> 
> The documentation explicitly states that "first close all virtual ports
> before closing the physical port" but we've found this to not always
> reality in our field situations. The GPRS / UTMS modem sometimes crashes
> and needs a power cycle in that case which means cleanly shutting down
> everything is not always possible. This change makes it much more robust
> for our situation where at least the system is recoverable with this patch
> and doesn't hang in a deadlock situation inside the kernel.
> 
> The patch is against the long term support kernel (3.4.27) and should
> apply cleanly to more recent branches. Tested with a Telit GE864-QUADV2
> and Telit HE910 modem.

It doesn't apply cleanly to the linux-next tree, can you refresh this
and resend it so that I can apply it?  If you want it backported to the
stable kernel releases, we can only do that after it gets into Linus's
tree, see the file, Documentation/stable_kernel_rules.txt for details
about this.

thanks,

greg k-h
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