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

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Message-ID: <5193D7E0.5010803@hurleysoftware.com>
Date:	Wed, 15 May 2013 14:45:52 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Joerg Roedel <joro@...tes.org>
CC:	Jiri Slaby <jslaby@...e.cz>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: Add missing lock in n_tty_write()

On 05/15/2013 11:47 AM, Joerg Roedel wrote:
> 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.

nack.

"space left" is not honored when OPOST is clear, so it is not protected
in this case. IOW, tty->ops->write_room() is not called, so by-definition
there is "space left".

Are you certain your stack trace takes you through this particular
invocation of tty->ops->write()?  Could it be that the compiler has
inlined process_output_block() into n_tty_write() and that's what your
seeing?

Can you attach the BUG report?
Are you certain OPOST is cleared? (output of stty -a -F </dev/xxxx>)
Is CONFIG_CONSOLE_POLL=y?
Is this happening during boot or much later?

> 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.

But not the only path to __write_console().

For example, what serializes hvc_console_print() with hvc_write()
for the same console index?

Regards,
Peter Hurley

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