commit b9bb53f3836f4eb2bdeb3447be11042bd29c2408
author Willem de Bruijn <willemb@google.com> Tue Apr 05 12:41:14 2016 -0400
committer David S. Miller <davem@davemloft.net> Tue Apr 05 16:29:36 2016 -0400
tree f1052dadfe3e5694c235c77c7447a75a7ec353c1
parent e43d15c8d3c8680fbf142360e5958f2ddd437047

sock: convert sk_peek_offset functions to WRITE_ONCE

Make the peek offset interface safe to use in lockless environments.
Use READ_ONCE and WRITE_ONCE to avoid race conditions between testing
and updating the peek offset.

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

diff --git a/include/net/sock.h b/include/net/sock.h
index 310c436..09aec75 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -459,26 +459,28 @@
 
 static inline int sk_peek_offset(struct sock *sk, int flags)
 {
-	if ((flags & MSG_PEEK) && (sk->sk_peek_off >= 0))
-		return sk->sk_peek_off;
-	else
-		return 0;
+	if (unlikely(flags & MSG_PEEK)) {
+		s32 off = READ_ONCE(sk->sk_peek_off);
+		if (off >= 0)
+			return off;
+	}
+
+	return 0;
 }
 
 static inline void sk_peek_offset_bwd(struct sock *sk, int val)
 {
-	if (sk->sk_peek_off >= 0) {
-		if (sk->sk_peek_off >= val)
-			sk->sk_peek_off -= val;
-		else
-			sk->sk_peek_off = 0;
+	s32 off = READ_ONCE(sk->sk_peek_off);
+
+	if (unlikely(off >= 0)) {
+		off = max_t(s32, off - val, 0);
+		WRITE_ONCE(sk->sk_peek_off, off);
 	}
 }
 
 static inline void sk_peek_offset_fwd(struct sock *sk, int val)
 {
-	if (sk->sk_peek_off >= 0)
-		sk->sk_peek_off += val;
+	sk_peek_offset_bwd(sk, -val);
 }
 
 /*