netdev - Re: [net-next PATCH 1/2] net: Busy polling should ignore sender CPUs

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Date:   Mon, 20 Mar 2017 15:16:11 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        sridhar.samudrala@...el.com, edumazet@...gle.com,
        davem@...emloft.net
Subject: Re: [net-next PATCH 1/2] net: Busy polling should ignore sender CPUs

On Mon, 2017-03-20 at 14:48 -0700, Alexander Duyck wrote:
> From: Alexander Duyck <alexander.h.duyck@...el.com>
> 
> This patch is a cleanup/fix for NAPI IDs following the changes that made it
> so that sender_cpu and napi_id were doing a better job of sharing the same
> location in the sk_buff.
> 
> One issue I found is that we weren't validating the napi_id as being valid
> before we started trying to setup the busy polling.  This change corrects
> that by using the MIN_NAPI_ID value that is now used in both allocating the
> NAPI IDs, as well as validating them.
> 
> Fixes: 52bd2d62ce675 ("net: better skb->sender_cpu and skb->napi_id cohabitation")
> Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.com>
> ---

This Fixes: tag seems not really needed here.

If really busy polling is attempted to a socket with a <wrong> napi id,
nothing bad happens. This fits the advisory model of busy polling...

Otherwise, your patch would be a candidate for net tree.

Also note that as soon as sk_can_busy_loop(sk) returns some status,
another cpu might already have changed sk->sk_napi_id to something else,
possibly with a <wrong> napi id again.

If your upcoming code depends on sk->sk_napi_id being verified, then
you need to read it once.