[PATCH net 2/2] net: core: take the reuseport migration reference with acquire ordering

reuseport_migrate_sock() returns a listener with a reference held, and
every caller immediately reads that listener: tcp_v4_rcv() and
tcp_v6_rcv() pass it to tcp_filter() and tcp_check_req(), and the
request-socket paths clone its state.  A listener removed from the
reuseport group concurrently with that lookup must not be observed in a
half-torn-down state after the reference was successfully taken.

refcount_inc_not_zero() lost its acquire ordering in the refcount_t
consolidation (fb041bb7c0a9), so these subsequent reads are not ordered
against the reference acquisition.  Use
refcount_inc_not_zero_acquire(), as was done for the established-socket
lookups, inet_twsk_purge() and bpf_sk_assign() in 5b5d8f351dd9.

No behaviour change on strongly ordered architectures.

Fixes: fb041bb7c0a9 ("locking/refcount: Consolidate implementations of refcount_t")
Cc: <redacted>
Signed-off-by: Hang Nan <redacted>
---
 net/core/sock_reuseport.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/core/sock_reuseport.c b/net/core/sock_reuseport.c
index fccbdd0430b0..78d1d7103122 100644
--- a/net/core/sock_reuseport.c
+++ b/net/core/sock_reuseport.c

@@ -666,7 +666,8 @@ struct sock *reuseport_migrate_sock(struct sock *sk,
 	if (!nsk)
 		nsk = reuseport_select_sock_by_hash(reuse, hash, socks);
 
-	if (IS_ERR_OR_NULL(nsk) || unlikely(!refcount_inc_not_zero(&nsk->sk_refcnt))) {
+	if (IS_ERR_OR_NULL(nsk) ||
+	    unlikely(!refcount_inc_not_zero_acquire(&nsk->sk_refcnt))) {
 		nsk = NULL;
 		goto failure;
 	}

-- 
2.53.0