Patch "rfs: annotate lockless accesses to sk->sk_rxhash" has been added to the 4.19-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    rfs: annotate lockless accesses to sk->sk_rxhash

to the 4.19-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     rfs-annotate-lockless-accesses-to-sk-sk_rxhash.patch
and it can be found in the queue-4.19 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 5b1bd1924ac01b5521345435d2db41dac3e86451
Author: Eric Dumazet <edumazet@xxxxxxxxxx>
Date:   Tue Jun 6 07:41:14 2023 +0000

    rfs: annotate lockless accesses to sk->sk_rxhash
    
    [ Upstream commit 1e5c647c3f6d4f8497dedcd226204e1880e0ffb3 ]
    
    Add READ_ONCE()/WRITE_ONCE() on accesses to sk->sk_rxhash.
    
    This also prevents a (smart ?) compiler to remove the condition in:
    
    if (sk->sk_rxhash != newval)
            sk->sk_rxhash = newval;
    
    We need the condition to avoid dirtying a shared cache line.
    
    Fixes: fec5e652e58f ("rfs: Receive Flow Steering")
    Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
    Reviewed-by: Simon Horman <simon.horman@xxxxxxxxxxxx>
    Reviewed-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
    Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/include/net/sock.h b/include/net/sock.h
index c140c6f86e4b1..616e84d1670df 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -988,8 +988,12 @@ static inline void sock_rps_record_flow(const struct sock *sk)
 		 * OR	an additional socket flag
 		 * [1] : sk_state and sk_prot are in the same cache line.
 		 */
-		if (sk->sk_state == TCP_ESTABLISHED)
-			sock_rps_record_flow_hash(sk->sk_rxhash);
+		if (sk->sk_state == TCP_ESTABLISHED) {
+			/* This READ_ONCE() is paired with the WRITE_ONCE()
+			 * from sock_rps_save_rxhash() and sock_rps_reset_rxhash().
+			 */
+			sock_rps_record_flow_hash(READ_ONCE(sk->sk_rxhash));
+		}
 	}
 #endif
 }
@@ -998,15 +1002,19 @@ static inline void sock_rps_save_rxhash(struct sock *sk,
 					const struct sk_buff *skb)
 {
 #ifdef CONFIG_RPS
-	if (unlikely(sk->sk_rxhash != skb->hash))
-		sk->sk_rxhash = skb->hash;
+	/* The following WRITE_ONCE() is paired with the READ_ONCE()
+	 * here, and another one in sock_rps_record_flow().
+	 */
+	if (unlikely(READ_ONCE(sk->sk_rxhash) != skb->hash))
+		WRITE_ONCE(sk->sk_rxhash, skb->hash);
 #endif
 }
 
 static inline void sock_rps_reset_rxhash(struct sock *sk)
 {
 #ifdef CONFIG_RPS
-	sk->sk_rxhash = 0;
+	/* Paired with READ_ONCE() in sock_rps_record_flow() */
+	WRITE_ONCE(sk->sk_rxhash, 0);
 #endif
 }