netdev - Re: [PATCH - regression 4.1-rc8] can: fix loss of CAN frames in raw

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Date:	Mon, 22 Jun 2015 12:10:14 +0200
From:	Manfred Schlaegl <manfred.schlaegl@....at>
To:	Oliver Hartkopp <socketcan@...tkopp.net>, netdev@...r.kernel.org,
	davem@...emloft.net, linux-kernel@...r.kernel.org,
	mkl@...gutronix.de
Subject: Re: [PATCH - regression 4.1-rc8] can: fix loss of CAN frames in raw_rcv

Hello!

On 2015-06-21 18:50, Oliver Hartkopp wrote:
> As reported by Manfred Schlaegl here
> 
>    http://marc.info/?l=linux-netdev&m=143482089824232&w=2
> 
> commit 514ac99c64b "can: fix multiple delivery of a single CAN frame for
> overlapping CAN filters" requires the skb->tstamp to be set to check for
> identical CAN skbs.
> 
> As net timestamping is influenced by several players (netstamp_needed and
> netdev_tstamp_prequeue) Manfred missed a proper timestamp which leads to
> CAN frame loss.
> 
> As skb timestamping became now mandatory for CAN related skbs this patch
> makes sure that received CAN skbs always have a proper timestamp set.
> Maybe there's a better solution in the future but this patch fixes the
> CAN frame loss so far.
> 

I'm not sure, but maybe this patch (and also my original one) opens a new potential issue with timestamps.

If the timestamp is set at allocation time, this cancels setting the timestamp at delivery (by net_timestamp_check in, for example, netif_receive_skb_internal.) -> So it changes the behavior of timestamping (http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/networking/timestamping.txt?id=b953c0d234bc72e8489d3bf51a276c5c4ec85345) generally.

Hypothetical example: If timestamping is enabled by the user and there is a significant delay between allocation and delivery of a skb (early allocation in driver or something) the timestamp does not reflect the reception time anymore.

What do you thing about this?

best regards,
Manfred
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