Re: [Q] RCU usage in infiniband

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Jul 08, 2026 at 03:21:05PM +0200, Sebastian Andrzej Siewior wrote:
> On 2026-07-08 10:17:27 [-0300], Jason Gunthorpe wrote:
> > 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
> 
> Where? Within the module core or somewhere within infiniband?

Eg the uverbs path has a srcu synchronize IIRC before allowing the FDs
to close which holds the module as well. But it probably isn't wise to
rely on this

Jason