On Tue, 2 Sep 2014 17:39:30 -0400
Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> ---
> 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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