netdev - Re: [net-next 10/11] net/mlx5e: Implement queue mgmt ops and single channel swap

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Message-ID: <Z5ME2-zHJq6arJC8@x130>
Date: Thu, 23 Jan 2025 19:11:23 -0800
From: Saeed Mahameed <saeed@...nel.org>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Stanislav Fomichev <stfomichev@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Paolo Abeni <pabeni@...hat.com>, Eric Dumazet <edumazet@...gle.com>,
	Saeed Mahameed <saeedm@...dia.com>, netdev@...r.kernel.org,
	Tariq Toukan <tariqt@...dia.com>, Gal Pressman <gal@...dia.com>,
	Leon Romanovsky <leonro@...dia.com>,
	Dragos Tatulea <dtatulea@...dia.com>
Subject: Re: [net-next 10/11] net/mlx5e: Implement queue mgmt ops and single
 channel swap

On 23 Jan 16:55, Jakub Kicinski wrote:
>On Thu, 23 Jan 2025 16:39:05 -0800 Stanislav Fomichev wrote:
>> > > What technical debt accrued ? I haven't seen any changes in queue API since
>> > > bnxt and gve got merged, what changed since then ?
>> > >
>> > > mlx5 doesn't require rtnl if this is because of the assert, I can remove
>> > > it. I don't understand what this series is being deferred for, please
>> > > elaborate, what do I need to do to get it accepted ?
>> >
>> > Remove the dependency on rtnl_lock _in the core kernel_.
>>
>> IIUC, we want queue API to move away from rtnl and use only (new) netdev
>> lock. Otherwise, removing this dependency in the future might be
>> complicated.
>
>Correct. We only have one driver now which reportedly works (gve).
>Let's pull queues under optional netdev_lock protection.
>Then we can use queue mgmt op support as a carrot for drivers
>to convert / test the netdev_lock protection... "compliance".
>
>I added netdev_lock protection for NAPI before the merge window.
>Queues are configured in much more ad-hoc fashion, so I think
>the best way to make queue changes netdev_lock safe would be to
>wrap all driver ops which are currently under rtnl_lock with
>netdev_lock.

Are you expecting drivers to hold netdev_lock internally? 
I was thinking something more scalable, queue_mgmt API to take
netdev_lock,  and any other place in the stack that can access 
"netdev queue config" e.g ethtool/netlink/netdev_ops should grab 
netdev_lock as well, this is better for the future when we want to 
reduce rtnl usage in the stack to protect single netdev ops where
netdev_lock will be sufficient, otherwise you will have to wait for ALL
drivers to properly use netdev_lock internally to even start thinking of
getting rid of rtnl from some parts of the core stack.