netdev - Re: [PATCH v3] ucc_geth: Lockless xmit

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Message-ID: <20120921125129.GA21792@electric-eye.fr.zoreil.com>
Date:	Fri, 21 Sep 2012 14:51:29 +0200
From:	Francois Romieu <romieu@...zoreil.com>
To:	Joakim Tjernlund <Joakim.Tjernlund@...nsmode.se>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH v3] ucc_geth: Lockless xmit

Joakim Tjernlund <Joakim.Tjernlund@...nsmode.se> :
> Currently ucc_geth_start_xmit wraps IRQ off for the
> whole body just to be safe. By rearranging the code a bit
> one can avoid the lock completely.

Afaics you went a bit too lockless with the queueing disable / enable
logic. The hard_start_xmit handler is run in a locally softirq disabled
section but it will happily race with the napi handler on a different
CPU. Grep netif_tx_lock in tg3.c for it.

The Tx skb free logic probably requires some smp memory barriers as
well since the current skb is used by the ucc_geth driver to sync the
Tx xmit with the napi completion handler.

-- 
Ueimor
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