Re: [PATCH v2 3/8] fs: lockless mntns rbtree lookup

[Christian Brauner <brauner@kernel.org>]

On Thu, Dec 12, 2024 at 01:09:29PM +0100, Peter Zijlstra wrote:
> On Thu, Dec 12, 2024 at 12:56:02PM +0100, Christian Brauner wrote:
> 
> 
> > @@ -16,7 +16,10 @@ struct mnt_namespace {
> >  	u64 event;
> >  	unsigned int		nr_mounts; /* # of mounts in the namespace */
> >  	unsigned int		pending_mounts;
> > -	struct rb_node		mnt_ns_tree_node; /* node in the mnt_ns_tree */
> > +	union {
> > +		struct rb_node	mnt_ns_tree_node; /* node in the mnt_ns_tree */
> > +		struct rcu_head mnt_ns_rcu;
> > +	};
> >  	refcount_t		passive; /* number references not pinning @mounts */
> >  } __randomize_layout;
> 
> >  static void mnt_ns_tree_remove(struct mnt_namespace *ns)
> >  {
> >  	/* remove from global mount namespace list */
> >  	if (!is_anon_ns(ns)) {
> > -		guard(write_lock)(&mnt_ns_tree_lock);
> > +		mnt_ns_tree_write_lock();
> >  		rb_erase(&ns->mnt_ns_tree_node, &mnt_ns_tree);
> > +		mnt_ns_tree_write_unlock();
> >  	}
> >  
> > -	mnt_ns_release(ns);
> > +	call_rcu(&ns->mnt_ns_rcu, mnt_ns_release_rcu);
> >  }
> 
> I'm not sure that union is sane; the above means you're overwriting the
> tree node while a concurrent lookup might still see the node and want to
> decent from it.

Though, you're right. I'll fix this up. Thanks for the reviews!