netdev - Re: [PATCH v2 net 2/8] net: mscc: ocelot: add locking for the port TX timestamp ID

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Message-ID: <51c3dc0a-6433-185b-ec9a-1622879965c7@gmail.com>
Date:   Thu, 17 Sep 2020 19:37:25 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>, davem@...emloft.net,
        netdev@...r.kernel.org
Cc:     yangbo.lu@....com, xiaoliang.yang_1@....com,
        UNGLinuxDriver@...rochip.com, claudiu.manoil@....com,
        alexandre.belloni@...tlin.com, andrew@...n.ch,
        vivien.didelot@...il.com, kuba@...nel.org
Subject: Re: [PATCH v2 net 2/8] net: mscc: ocelot: add locking for the port TX
 timestamp ID



On 9/17/2020 6:07 PM, Vladimir Oltean wrote:
> From: Vladimir Oltean <vladimir.oltean@....com>
> 
> The ocelot_port->ts_id is used to:
> (a) populate skb->cb[0] for matching the TX timestamp in the PTP IRQ
>      with an skb.
> (b) populate the REW_OP from the injection header of the ongoing skb.
> Only then is ocelot_port->ts_id incremented.
> 
> This is a problem because, at least theoretically, another timestampable
> skb might use the same ocelot_port->ts_id before that is incremented.
> Normally all transmit calls are serialized by the netdev transmit
> spinlock, but in this case, ocelot_port_add_txtstamp_skb() is also
> called by DSA, which has started declaring the NETIF_F_LLTX feature
> since commit 2b86cb829976 ("net: dsa: declare lockless TX feature for
> slave ports").  So the logic of using and incrementing the timestamp id
> should be atomic per port.
> 
> The solution is to use the global ocelot_port->ts_id only while
> protected by the associated ocelot_port->ts_id_lock. That's where we
> populate skb->cb[0]. Note that for ocelot, ocelot_port_add_txtstamp_skb
> is called for the actual skb, but for felix, it is called for the skb's
> clone. That is something which will also be changed in the future.
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
> Reviewed-by: Horatiu Vultur <horatiu.vultur@...rochip.com>

Reviewed-by: Florian Fainelli <f.fainelli@...il.com>
-- 
Florian