On Fri, 24 Mar 2017 07:05:12 +0800
gfree.w...@foxmail.com wrote:
> From: Gao Feng <f...@ikuai8.com>
>
> Because sysctl_tcp_adv_win_scale could be changed any time, so there
> is one race in tcp_win_from_space.
> For example,
> 1.sysctl_tcp_adv_win_scale<=0 (sysctl_tcp_adv_win_scale is negative now)
> 2.space>>(-sysctl_tcp_adv_win_scale) (sysctl_tcp_adv_win_scale is postive
> now)
>
> As a result, tcp_win_from_space returns 0. It is unexpected.
>
> Certainly if the compiler put the sysctl_tcp_adv_win_scale into one
> register firstly, then use the register directly, it would be ok.
> But we could not depend on the compiler behavior.
>
> Signed-off-by: Gao Feng <f...@ikuai8.com>
> ---
> include/net/tcp.h | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 6ec4ea6..119b592 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -1252,9 +1252,11 @@ void tcp_select_initial_window(int __space, __u32 mss,
> __u32 *rcv_wnd,
>
> static inline int tcp_win_from_space(int space)
> {
> - return sysctl_tcp_adv_win_scale<=0 ?
> - (space>>(-sysctl_tcp_adv_win_scale)) :
> - space - (space>>sysctl_tcp_adv_win_scale);
> + int tcp_adv_win_scale = sysctl_tcp_adv_win_scale;
> +
> + return tcp_adv_win_scale <= 0 ?
> + (space>>(-tcp_adv_win_scale)) :
> + space - (space>>tcp_adv_win_scale);
> }
>
> /* Note: caller must be prepared to deal with negative returns */
You need to use READ_ONCE() to be safe. The compiler is free to optimized the
code back
to the original unless READ_ONCE() is used.
