> -----Original Message----- > From: netdev-ow...@vger.kernel.org [mailto:netdev-ow...@vger.kernel.org] > On Behalf Of Stephen Hemminger > Sent: Saturday, March 25, 2017 4:32 AM > To: gfree.w...@foxmail.com > Cc: da...@davemloft.net; netdev@vger.kernel.org; Gao Feng > <f...@ikuai8.com> > Subject: Re: [PATCH net-next 1/1] tcp: sysctl: Fix a race to avoid unexpected 0 > window from space > > 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.
Thanks your reminder. I could not figure out why compiler would do the optimization, rollback the global variable instead of local tmp variable. It breaks the original logic. BTW, I think the READ_ONCE is used to avoid reorder. Could you give some guides please ? Best Regards Feng
