Re: bit fields && data tearing

Benjamin Herrenschmidt
2014-09-03T23:11:33Z

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 <peter@hurleysoftware.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;