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

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Message-ID: <Z5PrXkL7taguM57W@x130>
Date: Fri, 24 Jan 2025 11:34:54 -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 24 Jan 07:26, Jakub Kicinski wrote:
>On Thu, 23 Jan 2025 19:11:23 -0800 Saeed Mahameed wrote:
>> On 23 Jan 16:55, Jakub Kicinski wrote:
>> >> 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.
>
>Agreed, expecting drivers to get the locking right internally is easier
>short term but messy long term. I'm thinking opt-in for drivers to have
>netdev_lock taken by the core. Probably around all ops which today hold
>rtnl_lock, to keep the expectations simple.
>

Why opt-in? I don't see any overhead of taking netdev_lock by default in
rtnl_lock flows.

>net_shaper and queue_mgmt ops can require that drivers that support
>them opt-in and these ops can hold just the netdev_lock, no rtnl_lock.