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

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Message-ID: <20220627104945.5d8337a5@kernel.org>
Date:   Mon, 27 Jun 2022 10:49:45 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Ido Schimmel <idosch@...dia.com>, Jiri Pirko <jiri@...nulli.us>
Cc:     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

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
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
be complete in the next two months.