[PATCH] gianfar: Fix TX ring processing on SMP machines
Kumar Gala
galak at kernel.crashing.org
Fri Mar 5 03:34:15 EST 2010
On Mar 4, 2010, at 2:41 AM, David Miller wrote:
> From: Anton Vorontsov <avorontsov at ru.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 at ge.com>
>> Bisected-by: Paul Gortmaker <paul.gortmaker at windriver.com>
>> Signed-off-by: Anton Vorontsov <avorontsov at ru.mvista.com>
>> Tested-by: Paul Gortmaker <paul.gortmaker at windriver.com>
>> Tested-by: Martyn Welch <martyn.welch at ge.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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