Re: [PATCH net-next 5/6] devlink: Reshuffle resource registration logic

From: Jakub Kicinski
Date: Mon Nov 22 2021 - 21:27:33 EST

On Sun, 21 Nov 2021 10:45:12 +0200 Leon Romanovsky wrote:
> On Fri, Nov 19, 2021 at 08:10:17AM -0800, Jakub Kicinski wrote:
> > On Fri, 19 Nov 2021 17:38:53 +0200 Leon Romanovsky wrote:
> > > My approach works, exactly like it works in other subsystems.
> > >
> >
> > What "other subsystems"? I'm aware of the RFC version of these patches.
> Approach to have fine-grained locking scheme, instead of having one big lock.
> This was done in MM for mmap_sem, we did it for RDMA too.

You're breaking things up to avoid lock ordering issues. The user can
still only run a single write command at a time.

> > Breaking up the locks to to protect sub-objects only is fine for
> > protecting internal lists but now you can't guarantee that the object
> > exists when driver is called.
> I can only guess about which objects you are talking.

It obviously refers to the port splitting I mentioned below.

> If you are talking about various devlink sub-objects (ports, traps,
> e.t.c), they created by the drivers and as such should be managed by them.
> Also they are connected to devlink which is guaranteed to exist. At the end,
> they called to devlink_XXX->devlink pointer without any existence check.
> If you are talking about devlink instance itself, we guarantee that it
> exists between devlink_alloc() and devlink_free(). It seems to me pretty
> reasonable request from drivers do not access devlink before devlink_alloc()
> or after devlink_free(),
> > I'm sure you'll utter your unprovable "in real drivers.." but the fact
> > is my approach does not suffer from any such issues. Or depends on
> > drivers registering devlink last.
> Registration of devlink doesn't do anything except opening it to the world.
> The lifetime is controlled with alloc and free. My beloved sentence "in
> real drivers ..." belongs to use of devlink_put and devlink_locks outside
> of devlink.c and nothing more.

As soon as there is a inter-dependency between two subsystems "must
be last" breaks down.

> > I can start passing a pointer to a devlink_port to split/unsplit
> > functions, which is a great improvement to the devlink driver API.
> You can do it with my approach too. We incremented reference counter
> of devlink instance when devlink_nl_cmd_port_split_doit() was called,
> and we can safely take devlink->port_list_lock lock before returning
> from pre_doit.

Wait, I thought you'd hold devlink->lock around split/unsplit.

Please look at the port splitting case, mlx5 doesn't implement it
but it's an important feature.

Either way, IDK how ref count on devlink helps with lifetime of a
subobject. You must assume the sub-objects can only be created outside
of the time devlink instance is visible or under devlink->lock?