netdev - Re: [PATCH AUTOSEL 5.15 41/46] net: dpaa2: publish MAC stringset to ethtool -S even if MAC is missing

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Message-ID: <Y6ZH4YCuBSiPDMNd@sashalap>
Date:   Fri, 23 Dec 2022 19:29:21 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Andrew Lunn <andrew@...n.ch>,
        Ioana Ciornei <ioana.ciornei@....com>,
        Paolo Abeni <pabeni@...hat.com>, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, netdev@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 5.15 41/46] net: dpaa2: publish MAC stringset to
 ethtool -S even if MAC is missing

On Mon, Dec 19, 2022 at 01:54:02PM +0200, Vladimir Oltean wrote:
>Hi Sasha,
>
>On Sun, Dec 18, 2022 at 11:12:39AM -0500, Sasha Levin wrote:
>> From: Vladimir Oltean <vladimir.oltean@....com>
>>
>> [ Upstream commit 29811d6e19d795efcf26644b66c4152abbac35a6 ]
>>
>> DPNIs and DPSW objects can connect and disconnect at runtime from DPMAC
>> objects on the same fsl-mc bus. The DPMAC object also holds "ethtool -S"
>> unstructured counters. Those counters are only shown for the entity
>> owning the netdev (DPNI, DPSW) if it's connected to a DPMAC.
>>
>> The ethtool stringset code path is split into multiple callbacks, but
>> currently, connecting and disconnecting the DPMAC takes the rtnl_lock().
>> This blocks the entire ethtool code path from running, see
>> ethnl_default_doit() -> rtnl_lock() -> ops->prepare_data() ->
>> strset_prepare_data().
>>
>> This is going to be a problem if we are going to no longer require
>> rtnl_lock() when connecting/disconnecting the DPMAC, because the DPMAC
>> could appear between ops->get_sset_count() and ops->get_strings().
>> If it appears out of the blue, we will provide a stringset into an array
>> that was dimensioned thinking the DPMAC wouldn't be there => array
>> accessed out of bounds.
>>
>> There isn't really a good way to work around that, and I don't want to
>> put too much pressure on the ethtool framework by playing locking games.
>> Just make the DPMAC counters be always available. They'll be zeroes if
>> the DPNI or DPSW isn't connected to a DPMAC.
>>
>> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
>> Reviewed-by: Andrew Lunn <andrew@...n.ch>
>> Reviewed-by: Ioana Ciornei <ioana.ciornei@....com>
>> Tested-by: Ioana Ciornei <ioana.ciornei@....com>
>> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
>> Signed-off-by: Sasha Levin <sashal@...nel.org>
>> ---
>
>I think the algorithm has a problem in that it has a tendency to
>auto-pick preparatory patches which eliminate limitations that are
>preventing future development from taking place, rather than patches
>which fix present issues in the given code base.

Yeah, I'd agree. I think that the tricky part is that preperatory
patches usually resolve an issue, but it's not clear whether it's
something that affects users, or is just a theoretical limitation needed
by future patches.

>In this case, the patch is part of a larger series which was at the
>boundary between "next" work and "stable" work (patch 07/12 of this)
>https://patchwork.kernel.org/project/netdevbpf/cover/20221129141221.872653-1-vladimir.oltean@nxp.com/
>
>Due to the volume of that rework, I intended it to go to "next", even
>though backporting the entire series to "stable" could have its own
>merits. But picking just patch 07/12 out of that series is pointless,
>so please drop this patch from the queue for 5.15, 6.0 and 6.1, please.

Now dropped, thanks!

-- 
Thanks,
Sasha