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Re: [net-next PATCH 1/5] net: Do not record sender_cpu as napi_id in socket receive paths

On 3/16/2017 3:05 PM, Eric Dumazet wrote:
On Thu, 2017-03-16 at 11:32 -0700, Alexander Duyck wrote:
From: Sridhar Samudrala <sridhar.samudrala@xxxxxxxxx>

Fix sk_mark_napi_id() and sk_mark_napi_id_once() to set sk_napi_id only if
skb->napi_id is a valid value.

This happens in loopback paths where skb->napi_id is not updated in
rx path and holds sender_cpu that is set in xmit path.

Signed-off-by: Sridhar Samudrala <sridhar.samudrala@xxxxxxxxx>
Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxx>
  include/net/busy_poll.h |    5 +++--
  1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/net/busy_poll.h b/include/net/busy_poll.h
index c0452de83086..67991635953e 100644
--- a/include/net/busy_poll.h
+++ b/include/net/busy_poll.h
@@ -116,7 +116,8 @@ static inline bool sk_busy_loop(struct sock *sk, int nonblock)
  static inline void sk_mark_napi_id(struct sock *sk, const struct sk_buff *skb)
-	sk->sk_napi_id = skb->napi_id;
+	if (skb->napi_id > (u32)NR_CPUS)
+		sk->sk_napi_id = skb->napi_id;
@@ -125,7 +126,7 @@ static inline void sk_mark_napi_id_once(struct sock *sk,
  					const struct sk_buff *skb)
-	if (!sk->sk_napi_id)
+	if (!sk->sk_napi_id && (skb->napi_id > (u32)NR_CPUS))
  		sk->sk_napi_id = skb->napi_id;

It is not clear why this patch is needed .

What you describe here is the case we might receive packets for a socket
coming from different interfaces ?

This is seen with AF_UNIX or AF_INET sockets over loopback.

If skb->napi_id is a sender_cpu, why should we prevent overwriting the
sk_napi_id with it, knowing that busy polling will simply ignore the
invalid value ?

We are not checking for invalid VALUE(< NR_CPUs) in busy_poll,
Non-zero sk->napi_id is considered valid.

If we don't want to add this check while setting sk->sk_napi_Id, we could change the check in ep_set_busy_poll_napi_id() to check for invalid value rather than non-zero value.

Do not get me wrong :

I simply try to understand why the test about napi_id validity is now
done twice :

1) At the time we are writing into sk->sk_napi_id

2) At busy polling time when we read sk->sk_napi_id