Re: [PATCH net] ipv6: sr: Add unlikely hint to idev NULL check in seg6_hmac_validate_skb

[Eric Dumazet]

On Wed, Mar 11, 2026 at 8:23 AM Minhong He <heminhong@xxxxxxxxxx> wrote:
>
> In seg6_hmac_validate_skb(), the pointer returned by __in6_dev_get()
> can theoretically be NULL if the network device lacks proper IPv6
> initialization or is being torn down. However, this is an exceptional
> error path; in normal operation, the device will always have a valid
> inet6_dev structure.
>
> Marking this NULL check with unlikely() helps the compiler optimize
> branch prediction and code layout. It ensures that the common case
> (valid idev) remains in the hot path, while the error handling code
> is placed out of line, improving CPU pipeline efficiency for high-speed
> SRv6 packet processing.

This section is unnecessary and dubious ?
Note that both clang and gcc already generate the correct code without
the unlikely()
So your claim about "CPU pipeline efficiency" is quite strange to me.

Let's keep the changelogs concise and focused on the real issue at hand.

Thanks.

> Fixes: bf355b8d2c30 ("ipv6: sr: add core files for SR HMAC support")
> Signed-off-by: Minhong He <heminhong@xxxxxxxxxx>
> ---
>  net/ipv6/seg6_hmac.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/net/ipv6/seg6_hmac.c b/net/ipv6/seg6_hmac.c
> index ee6bac0160ac..a8f279ffb678 100644
> --- a/net/ipv6/seg6_hmac.c
> +++ b/net/ipv6/seg6_hmac.c
> @@ -184,6 +184,8 @@ bool seg6_hmac_validate_skb(struct sk_buff *skb)
>         int require_hmac;
>
>         idev = __in6_dev_get(skb->dev);
> +       if (unlikely(!idev))
> +               return false;
>
>         srh = (struct ipv6_sr_hdr *)skb_transport_header(skb);
>
> --
> 2.25.1
>