linux-kernel - Re: [PATCH 21/26] tty: Convert tty

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Message-ID: <20140903115852.5a720754@alan.etchedpixels.co.uk>
Date:	Wed, 3 Sep 2014 11:58:52 +0100
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 21/26] tty: Convert tty_struct bitfield to bools

On Tue,  2 Sep 2014 17:39:30 -0400
Peter Hurley <peter@...leysoftware.com> wrote:

> The stopped, hw_stopped, flow_stopped and packet bits are smp-unsafe
> and interrupt-unsafe. For example,
> 
> CPU 0                         | CPU 1
>                               |
> tty->flow_stopped = 1         | tty->hw_stopped = 0
> 
> One of these updates will be corrupted, as the bitwise operation
> on the bitfield is non-atomic.
> 
> Ensure each flag has a separate memory location, so concurrent
> updates do not corrupt orthogonal states.

Ouch....

Alas the fix is not sufficient on some platforms. gcc will happily use
32bit operations on those fields if it thinks its a performance win.

It needs to use set_bit and friends.

x86 is generally ok, but ia64 gcc will do things like load all four
bytes, mask and write the dword back. I believe ARM gcc may also
sometimes generate similar results.


> 
> Signed-off-by: Peter Hurley <peter@...leysoftware.com>
> ---
>  include/linux/tty.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/tty.h b/include/linux/tty.h
> index 1c3316a..7cf61cb 100644
> --- a/include/linux/tty.h
> +++ b/include/linux/tty.h
> @@ -261,7 +261,10 @@ struct tty_struct {
>  	unsigned long flags;
>  	int count;
>  	struct winsize winsize;		/* winsize_mutex */
> -	unsigned char stopped:1, hw_stopped:1, flow_stopped:1, packet:1;
> +	bool stopped;
> +	bool hw_stopped;
> +	bool flow_stopped;
> +	bool packet;
>  	unsigned char ctrl_status;	/* ctrl_lock */
>  	unsigned int receive_room;	/* Bytes free for queue */
>  	int flow_change;
--
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