linux-kernel - Re: [PATCH net-next] devlink: Require devlink lock during device reload

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Date:   Mon, 15 Nov 2021 15:42:58 +0100
From:   Jiri Pirko <jiri@...nulli.us>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Leon Romanovsky <leon@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>, Jiri Pirko <jiri@...dia.com>,
        Ido Schimmel <idosch@...sch.org>,
        "David S . Miller" <davem@...emloft.net>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        edwin.peer@...adcom.com
Subject: Re: [PATCH net-next] devlink: Require devlink lock during device
 reload

Mon, Nov 15, 2021 at 01:53:59PM CET, jgg@...dia.com wrote:
>On Mon, Nov 15, 2021 at 12:20:21PM +0100, Jiri Pirko wrote:
>> Sun, Nov 14, 2021 at 07:19:02AM CET, leon@...nel.org wrote:
>> >On Fri, Nov 12, 2021 at 08:38:56AM +0100, Jiri Pirko wrote:
>> >> Thu, Nov 11, 2021 at 01:17:52PM CET, leon@...nel.org wrote:
>> >> >On Thu, Nov 11, 2021 at 01:05:11PM +0100, Jiri Pirko wrote:
>> >> >> Tue, Nov 09, 2021 at 07:24:27PM CET, jgg@...dia.com wrote:
>> >> >> >On Tue, Nov 09, 2021 at 08:20:42AM -0800, Jakub Kicinski wrote:
>> >> >> >> On Tue, 9 Nov 2021 11:33:35 -0400 Jason Gunthorpe wrote:
>> >> >> >> > > > I once sketched out fixing this by removing the need to hold the
>> >> >> >> > > > per_net_rwsem just for list iteration, which in turn avoids holding it
>> >> >> >> > > > over the devlink reload paths. It seemed like a reasonable step toward
>> >> >> >> > > > finer grained locking.  
>> >> >> >> > > 
>> >> >> >> > > Seems to me the locking is just a symptom.  
>> >> >> >> > 
>> >> >> >> > My fear is this reload during net ns destruction is devlink uAPI now
>> >> >> >> > and, yes it may be only a symptom, but the root cause may be unfixable
>> >> >> >> > uAPI constraints.
>> >> >> >> 
>> >> >> >> If I'm reading this right it locks up 100% of the time, what is a uAPI
>> >> >> >> for? DoS? ;)
>> >> >> >> 
>> >> >> >> Hence my questions about the actual use cases.
>> >> >> >
>> >> >> >Removing namespace support from devlink would solve the crasher. I
>> >> >> >certainly didn't feel bold enough to suggest such a thing :)
>> >> >> >
>> >> >> >If no other devlink driver cares about this it is probably the best
>> >> >> >idea.
>> >> >> 
>> >> >> Devlink namespace support is not generic, not related to any driver.
>> >> >
>> >> >What do you mean?
>> >> >
>> >> >devlink_pernet_pre_exit() calls to devlink reload, which means that only
>> >> >drivers that support reload care about it. The reload is driver thing.
>> >> 
>> >> However, Jason was talking about "namespace support removal from
>> >> devlink"..
>> >
>> >The code that sparkles deadlocks is in devlink_pernet_pre_exit() and
>> >this will be nice to remove. I just don't know if it is possible to do
>> >without ripping whole namespace support from devlink.
>> 
>> As discussed offline, the non-standard mlx5/IB usage of network
>> namespaces requires non standard mlx5/IB workaround. Does not make any
>> sense to remove the devlink net namespace support removal.
>
>Sorry, I don't agree that registering a net notifier in an aux device
>probe function is non-standard or wrong.


Listening to events which happen in different namespaces and react to
them is the non-standard behaviour which I refered to. If you would not
need to do it, you could just use netns notofier which would solve your
issue. You know it.


>
>This model must be supported sanely somehow in the netdev area and
>cannot be worked around in leaf drivers.
>
>Intel ice will have the same problem, as would broadcom if they ever
>get their driver modernized.
>
>Jason