Re: [Q] RCU usage in infiniband

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Jul 08, 2026 at 11:23:16AM +0200, Sebastian Andrzej Siewior wrote:
> Hi,
> 
> I've been randomly starting at infiniband code and noticed it uses
> call_rcu() with a callback from its module. 
> 
> | drivers/infiniband/hw/mlx5/devx.c:         call_rcu(&event_sub->rcu, devx_free_subscription);
> | drivers/infiniband/ulp/ipoib/ipoib_main.c: call_rcu(&neigh->rcu, ipoib_neigh_reclaim);
> 
> I don't see synchronize_rcu() and rcu_barrier() there so I've been
> asking myself what ensures that the callback completes before module is
> gone (via rmmod)? I would expect a rcu_barrier() in
> ipoib_cleanup_module() for instance.
> 
> Is the unload path so "late" that the callbacks run before the module is
> unmapped or is there something between the APIs that ensures this?

It wouldn't hurt to have a more explicit unload synchronize call, but
there are already some existing ones on the path to unload a module
that would cover anyhow

Jason