Subject: Re: bit fields && data tearing

From: Benjamin Herrenschmidt <[email-masked]>
Date: Thu, 04 Sep 2014 09:11:33 +1000

On Wed, 2014-09-03 at 18:51 -0400, Peter Hurley wrote:

> Apologies for hijacking this thread but I need to extend this discussion
> somewhat regarding what a compiler might do with adjacent fields in a structure.
> 
> The tty subsystem defines a large aggregate structure, struct tty_struct.
> Importantly, several different locks apply to different fields within that
> structure; ie., a specific spinlock will be claimed before updating or accessing
> certain fields while a different spinlock will be claimed before updating or
> accessing certain _adjacent_ fields.
> 
> What is necessary and sufficient to prevent accidental false-sharing?
> The patch below was flagged as insufficient on ia64, and possibly ARM.

We expect native aligned scalar types to be accessed atomically (the
read/modify/write of a larger quantity that gcc does on some bitfield
cases has been flagged as a gcc bug, but shouldn't happen on normal
scalar types).

I am not 100% certain of "bool" here, I assume it's treated as a normal
scalar and thus atomic but if unsure, you can always use int.

Another option is to use the atomic bitops and make these bits in a
bitmask but that is probably unnecessary if you have locks already.

Cheers,
Ben.

> Regards,
> Peter Hurley
> 
> --- >% ---
> Subject: [PATCH 21/26] tty: Convert tty_struct bitfield to bools
> 
> 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.
> 
> Signed-off-by: Peter Hurley <[email-masked]>
> ---
>  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;