netdev - Re: [PATCH v2 net] net: dsa: declare lockless TX feature for slave ports

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Message-Id: <20200527.151004.24719580946330092.davem@davemloft.net>
Date:   Wed, 27 May 2020 15:10:04 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     olteanv@...il.com
Cc:     andrew@...n.ch, f.fainelli@...il.com, vivien.didelot@...il.com,
        xiyou.wangcong@...il.com, ap420073@...il.com,
        netdev@...r.kernel.org
Subject: Re: [PATCH v2 net] net: dsa: declare lockless TX feature for slave
 ports

From: Vladimir Oltean <olteanv@...il.com>
Date: Wed, 27 May 2020 21:08:05 +0300

> From: Vladimir Oltean <vladimir.oltean@....com>
> 
> Be there a platform with the following layout:
> 
>       Regular NIC
>        |
>        +----> DSA master for switch port
>                |
>                +----> DSA master for another switch port
> 
> After changing DSA back to static lockdep class keys in commit
> 1a33e10e4a95 ("net: partially revert dynamic lockdep key changes"), this
> kernel splat can be seen:
> 
> [   13.361198] ============================================
> [   13.366524] WARNING: possible recursive locking detected
> [   13.371851] 5.7.0-rc4-02121-gc32a05ecd7af-dirty #988 Not tainted
> [   13.377874] --------------------------------------------
> [   13.383201] swapper/0/0 is trying to acquire lock:
> [   13.388004] ffff0000668ff298 (&dsa_slave_netdev_xmit_lock_key){+.-.}-{2:2}, at: __dev_queue_xmit+0x84c/0xbe0
> [   13.397879]
> [   13.397879] but task is already holding lock:
> [   13.403727] ffff0000661a1698 (&dsa_slave_netdev_xmit_lock_key){+.-.}-{2:2}, at: __dev_queue_xmit+0x84c/0xbe0
 ...
> There appears to be no negative side-effect to declaring lockless TX for
> the DSA virtual interfaces, which means they handle their own locking.
> So that's what we do to make the splat go away.
> 
> Patch tested in a wide variety of cases: unicast, multicast, PTP, etc.
> 
> Fixes: ab92d68fc22f ("net: core: add generic lockdep keys")
> Suggested-by: Taehee Yoo <ap420073@...il.com>
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
> Reviewed-by: Florian Fainelli <f.fainelli@...il.com>

Applied, thanks.