netdev - [PATCH net-next 2/2] ipv6: remove one read_lock()/read_unlock() pair in rt6_check

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Message-ID: <20230313201732.887488-3-edumazet@google.com>
Date:   Mon, 13 Mar 2023 20:17:32 +0000
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org,
        eric.dumazet@...il.com, Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next 2/2] ipv6: remove one read_lock()/read_unlock() pair
 in rt6_check_neigh()

rt6_check_neigh() uses read_lock() to protect n->nud_state reading.

This seems overkill and causes false sharing.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/ipv6/route.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 25c00c6f5131c55055f30348ef4605f50440ddbb..e829bd880384077027dc79fcae1a62eb0073f7c8 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -687,16 +687,16 @@ static enum rt6_nud_state rt6_check_neigh(const struct fib6_nh *fib6_nh)
 	neigh = __ipv6_neigh_lookup_noref(fib6_nh->fib_nh_dev,
 					  &fib6_nh->fib_nh_gw6);
 	if (neigh) {
-		read_lock(&neigh->lock);
-		if (neigh->nud_state & NUD_VALID)
+		u8 nud_state = READ_ONCE(neigh->nud_state);
+
+		if (nud_state & NUD_VALID)
 			ret = RT6_NUD_SUCCEED;
 #ifdef CONFIG_IPV6_ROUTER_PREF
-		else if (!(neigh->nud_state & NUD_FAILED))
+		else if (!(nud_state & NUD_FAILED))
 			ret = RT6_NUD_SUCCEED;
 		else
 			ret = RT6_NUD_FAIL_PROBE;
 #endif
-		read_unlock(&neigh->lock);
 	} else {
 		ret = IS_ENABLED(CONFIG_IPV6_ROUTER_PREF) ?
 		      RT6_NUD_SUCCEED : RT6_NUD_FAIL_DO_RR;
-- 
2.40.0.rc1.284.g88254d51c5-goog