netdev - Re: [PATCH] gianfar: Fix TX ring processing on SMP machines

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Date:	Thu, 04 Mar 2010 00:41:57 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	avorontsov@...mvista.com
Cc:	martyn.welch@...com, paul.gortmaker@...driver.com,
	Sandeep.Kumar@...escale.com, galak@...nel.crashing.org,
	linuxppc-dev@...abs.org, netdev@...r.kernel.org
Subject: Re: [PATCH] gianfar: Fix TX ring processing on SMP machines

From: Anton Vorontsov <avorontsov@...mvista.com>
Date: Wed, 3 Mar 2010 21:18:58 +0300

> Starting with commit a3bc1f11e9b867a4f49505 ("gianfar: Revive SKB
> recycling") gianfar driver sooner or later stops transmitting any
> packets on SMP machines.
> 
> start_xmit() prepares new skb for transmitting, generally it does
> three things:
> 
> 1. sets up all BDs (marks them ready to send), except the first one.
> 2. stores skb into tx_queue->tx_skbuff so that clean_tx_ring()
>    would cleanup it later.
> 3. sets up the first BD, i.e. marks it ready.
> 
> Here is what clean_tx_ring() does:
> 
> 1. reads skbs from tx_queue->tx_skbuff
> 2. checks if the *last* BD is ready. If it's still ready [to send]
>    then it it isn't transmitted, so clean_tx_ring() returns.
>    Otherwise it actually cleanups BDs. All is OK.
> 
> Now, if there is just one BD, code flow:
> 
> - start_xmit(): stores skb into tx_skbuff. Note that the first BD
>   (which is also the last one) isn't marked as ready, yet.
> - clean_tx_ring(): sees that skb is not null, *and* its lstatus
>   says that it is NOT ready (like if BD was sent), so it cleans
>   it up (bad!)
> - start_xmit(): marks BD as ready [to send], but it's too late.
> 
> We can fix this simply by reordering lstatus/tx_skbuff writes.
> 
> Reported-by: Martyn Welch <martyn.welch@...com>
> Bisected-by: Paul Gortmaker <paul.gortmaker@...driver.com>
> Signed-off-by: Anton Vorontsov <avorontsov@...mvista.com>
> Tested-by: Paul Gortmaker <paul.gortmaker@...driver.com>
> Tested-by: Martyn Welch <martyn.welch@...com>

Applied.
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