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

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Date:   Tue, 28 Jun 2022 10:43:26 +0300
From:   Ido Schimmel <idosch@...dia.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        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, Jun 27, 2022 at 05:55:06PM +0200, Jiri Pirko wrote:
> Mon, Jun 27, 2022 at 05:41:31PM CEST, idosch@...dia.com wrote:
> >On Mon, Jun 27, 2022 at 03:54:59PM +0200, Jiri Pirko wrote:
> >> From: Jiri Pirko <jiri@...dia.com>
> >> 
> >> 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.
> 
> You are correct, that is missing. For me, as a side effect this patchset
> resolved the deadlock for LC auxdev you pointed out. That was my
> motivation for this patchset :)

Given that devlink does not enable "parallel_ops" and that the generic
netlink mutex is held throughout all callbacks, what prevents you from
simply dropping the devlink mutex now? IOW, why can't this series be
patch #1 and another patch that removes the devlink mutex?

> 
> 
> >
> >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.
> >
> >> 
> >> The first patch makes sure that the xarray that holds devlink pointers
> >> is possible to be safely iterated.
> >> 
> >> The second patch moves the user command mutex to be per-devlink.
> >> 
> >> Jiri Pirko (2):
> >>   net: devlink: make sure that devlink_try_get() works with valid
> >>     pointer during xarray iteration
> >>   net: devlink: replace devlink_mutex by per-devlink lock
> >> 
> >>  net/core/devlink.c | 256 ++++++++++++++++++++++++++++-----------------
> >>  1 file changed, 161 insertions(+), 95 deletions(-)
> >> 
> >> -- 
> >> 2.35.3
> >>