RE: [PATCH net-next 1/1] tcp: sysctl: Fix a race to avoid unexpected 0 window from space

-----Original Message-----
From: netdev-owner@vger.kernel.org [mailto:netdev-owner@vger.kernel.org]
On Behalf Of Stephen Hemminger
Sent: Saturday, March 25, 2017 4:32 AM
To: gfree.wind@foxmail.com
Cc: davem@davemloft.net; netdev@vger.kernel.org; Gao Feng
[off-list ref]
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.wind@foxmail.com wrote:

quoted

From: Gao Feng <redacted>

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 <redacted>
---
 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