Re: [PATCH] mm: backing-dev: use kfree_rcu() instead of synchronize_rcu_expedited()

[Christoph Hellwig]

On Fri, Oct 15, 2021 at 01:35:56PM +0100, Matthew Wilcox wrote:
>  struct backing_dev_info {
>         u64 id;
> -       struct rb_node rb_node; /* keyed by ->id */
> +       union {
> +               struct rb_node rb_node; /* keyed by ->id */
> +               struct rcu_head rcu;
> +       };
>         struct list_head bdi_list;
>         unsigned long ra_pages; /* max readahead in PAGE_SIZE units */
>         unsigned long io_pages; /* max allowed IO size */
> 
> 
> Christoph, independent of the inode lifetime problem, this actually seems
> like a good approach to take.  I don't see why we should synchronize_rcu()
> here?  Adding Jens (original introducer of the synchronize_rcu()), Mikulas
> (converted it to use _expedited) and Tejun (worked around a problem when
> using _expedited).

The kfree+rcu + your suggestion does seem like a good idea in general to
me.  But I'd still like to fix the actual bug being reported before
optimizing the area in a way that papers over the bug.