netdev - Re: [PATCH net 2/7] net: mscc: ocelot: add locking for the port TX timestamp ID

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Message-ID: <20200917234322.qqwxyq6k33qm6bca@skbuf>
Date:   Fri, 18 Sep 2020 02:43:40 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, yangbo.lu@....com,
        xiaoliang.yang_1@....com, UNGLinuxDriver@...rochip.com,
        claudiu.manoil@....com, alexandre.belloni@...tlin.com,
        andrew@...n.ch, vivien.didelot@...il.com, f.fainelli@...il.com,
        kuba@...nel.org
Subject: Re: [PATCH net 2/7] net: mscc: ocelot: add locking for the port TX
 timestamp ID

On Wed, Sep 16, 2020 at 05:19:26PM -0700, David Miller wrote:
> From: Vladimir Oltean <olteanv@...il.com>
> Date: Tue, 15 Sep 2020 21:22:24 +0300
>
> > This is a problem because, at least theoretically, another timestampable
> > skb might use the same ocelot_port->ts_id before that is incremented. So
> > the logic of using and incrementing the timestamp id should be atomic
> > per port.
>
> Have you actually observed this race in practice?
>
> All transmit calls are serialized by the netdev transmit spinlock.
>
> Let's not add locking if it is not actually necessary.

It's a bit more complicated.
This code is also used from DSA, and DSA now declares NETIF_F_LLTX.