netdev - Re: [patch net-next RFC 0/2] net: devlink: remove devlink big lock

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Date:   Tue, 28 Jun 2022 08:32:49 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Ido Schimmel <idosch@...dia.com>, netdev@...r.kernel.org,
        davem@...emloft.net, petrm@...dia.com, pabeni@...hat.com,
        edumazet@...gle.com, mlxsw@...dia.com, saeedm@...dia.com
Subject: Re: [patch net-next RFC 0/2] net: devlink: remove devlink big lock

Mon, Jun 27, 2022 at 07:49:45PM CEST, kuba@...nel.org wrote:
>On Mon, 27 Jun 2022 18:41:31 +0300 Ido Schimmel wrote:
>> On Mon, Jun 27, 2022 at 03:54:59PM +0200, Jiri Pirko wrote:
>> > This is an attempt to remove use of devlink_mutex. This is a global lock
>> > taken for every user command. That causes that long operations performed
>> > on one devlink instance (like flash update) are blocking other
>> > operations on different instances.  
>> 
>> This patchset is supposed to prevent one devlink instance from blocking
>> another? Devlink does not enable "parallel_ops", which means that the
>> generic netlink mutex is serializing all user space operations. AFAICT,
>> this series does not enable "parallel_ops", so I'm not sure what
>> difference the removal of the devlink mutex makes.
>> 
>> The devlink mutex (in accordance with the comment above it) serializes
>> all user space operations and accesses to the devlink devices list. This
>> resulted in a AA deadlock in the previous submission because we had a
>> flow where a user space operation (which acquires this mutex) also tries
>> to register / unregister a nested devlink instance which also tries to
>> acquire the mutex.
>> 
>> As long as devlink does not implement "parallel_ops", it seems that the
>> devlink mutex can be reduced to only serializing accesses to the devlink
>> devices list, thereby eliminating the deadlock.
>
>I'm unclear on why we can't wait for mlx5 locking rework which will

Sure we can, no rush.

>allow us to move completely to per-instance locks. Do you have extra
>insights into how that work is progressing? I was hoping that it will

It's under internal review afaik.

>be complete in the next two months. 

What do you mean exactly? Is that that we would be okay just with
devlink->lock? I don't think so. We need user lock because we can't take
devlink->lock for port split and reload. devlink_mutex protects that now,
the devlink->cmd_lock I'm introducing here just replaces devlink_mutex.
If we can do without, that is fine. I just can't see how.
Also, I don't see the relation to mlx5 work. What is that?