linux-kernel - Re: [PATCH] tty: Add missing lock in n_tty

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Message-ID: <20130515154753.GJ24440@8bytes.org>
Date:	Wed, 15 May 2013 17:47:53 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: Add missing lock in n_tty_write()

On Wed, May 15, 2013 at 05:03:17PM +0200, Jiri Slaby wrote:
> On 05/15/2013 12:56 PM, Joerg Roedel wrote:

> Are you fixing any bug here? output_lock does not protect
> tty->ops->write on the other places, not tty->ops->write.

Yes, I am trying to fix a BUG_ON that triggered in
drivers/tty/hvc/hvc_xen.c in function __write_console().  This function
was called from the place I am patching in this fix. My current
explanation for that BUG_ON is a race condition that comes
from concurrent calls to that function.

That is also the only explanation that makes sense because the
__write_console() function itself makes sure that the condition can not
hit.

In the comment for the n_tty_write function there is this remark:

 *      Locking: output_lock to protect column state and space left
 *               (note that the process_output*() functions take this
 *                lock themselves)

So the space left is managed in the ->write callback and needs
protection.

The process_output*() functions all (unless I am missing something)
take the output_lock before calling the tty->ops->write (directly or
indirectly).

The place I patched here is the only place in n_tty_write where the
->write call-back is invoked directly, and it happens without taking the
lock. I think this is a problem.

	Joerg

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