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

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Message-Id: <DF5053B4-DCD0-4B90-B9D7-E80D0C76CFAE@kernel.crashing.org>
Date:	Thu, 4 Mar 2010 10:34:15 -0600
From:	Kumar Gala <galak@...nel.crashing.org>
To:	Anton Vorontsov <avorontsov@...mvista.com>
Cc:	Martyn Welch <martyn.welch@...com>,
	David Miller <davem@...emloft.net>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Sandeep Gopalpet <Sandeep.Kumar@...escale.com>,
	linuxppc-dev list <linuxppc-dev@...abs.org>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] gianfar: Fix TX ring processing on SMP machines


On Mar 4, 2010, at 2:41 AM, David Miller wrote:

> 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.

Anton,

Once this makes it into Linus's tree can you make sure we get it added to -stable.

- k--
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