netdev - Re: [PATCH 04/10] net/macb: Fix a race in macb_start

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Date:	Thu, 6 Sep 2012 16:52:45 +0200
From:	Nicolas Ferre <nicolas.ferre@...el.com>
To:	David Miller <davem@...emloft.net>
CC:	<netdev@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
	<havard@...nnemoen.net>, <plagnioj@...osoft.com>,
	<jamie@...ieiles.com>, <linux-kernel@...r.kernel.org>,
	<patrice.vilchez@...el.com>
Subject: Re: [PATCH 04/10] net/macb: Fix a race in macb_start_xmit()

On 09/05/2012 11:30 PM, David Miller :
> From: Nicolas Ferre <nicolas.ferre@...el.com>
> Date: Wed, 5 Sep 2012 10:19:11 +0200
> 
>> From: Havard Skinnemoen <havard@...nnemoen.net>
>>
>> Fix a race in macb_start_xmit() where we unconditionally set the TSTART bit.
>> If an underrun just happened (we do this with interrupts disabled, so it might
>> not have been handled yet), the controller starts transmitting from the first
>> entry in the ring, which is usually wrong.
>> Restart the controller after error handling.
>>
>> Signed-off-by: Havard Skinnemoen <havard@...nnemoen.net>
>> [nicolas.ferre@...el.com: split patch in topics]
>> Signed-off-by: Nicolas Ferre <nicolas.ferre@...el.com>
> 
> Accumulating special case code and checks into the hot path of TX packet
> processing is extremely unwise.
> 
> Instead, when you handle the TX error conditions and reset the chip you
> should first ensure that there are no flows of control in the transmit
> function of your driver by using the appropriate locking et al. facilities.
> 
> For example, you can quiesce the transmit path by handling the chip error
> interrupt as follows:
> 
> 1) Disable chip interrupt generation.
> 
> 2) Schedule a workqueue so you can process the reset outside of hard
>    interrupt context.
> 
> 3) In the workqueue function, disable NAPI and perform a
>    netif_tx_disable() to guarentee there are no threads of
>    execution trying to queue up packets for TX into the driver.
> 
> 4) Perform your chip reset and whatever else is necessary.
> 
> 5) Re-enable NAPI and TX.
> 
> Then you don't need any special checks in your xmit method at all.

I see... I will rework the series and try to implement this as part of
the "[PATCH 06/10] net/macb: better manage tx errors"

So this patch will disappear in future v2 series and patch 06 will be
seriously modified. In fact I will also try to stack "cosmetic" patches
at the beginning of the series.

Thanks, best regards,
-- 
Nicolas Ferre
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