Re: [PATCH] fix bitmap corruption on close_range() with CLOSE_RANGE_UNSHARE

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Aug 06, 2024 at 10:41:59AM +0200, Christian Brauner wrote:

> > Would rcu_assign_pointer of pointers + smp_store_release of max_fds on expand
> > (all under ->files_lock, etc.) paired with
> > smp_load_acquire of max_fds + rcu_dereference of ->fd on file lookup side
> > be enough, or do we need an explicit smp_wmb/smp_rmb in there?
> 
> Afair, smp_load_acquire() would be a barrier for both later loads and
> stores and smp_store_release() would be a barrier for both earlier loads
> and stores.
> 
> Iiuc, here we only care about ordering stores to ->fd and max_fds on the
> write side and about ordering loads of max_fds and ->fd on the reader
> side. The reader doesn't actually write anything.
> 
> In other words, we want to make ->fd visible before max_fds on the write
> side and we want to load max_fds after ->fd.
> 
> So I think smp_wmb() and smp_rmb() would be sufficient. I also find it
> clearer in this case.

It's not the question of sufficiency; it's whether anything cheaper can be
had.