Re: [PATCH v3 03/10] fs: lockless mntns rbtree lookup

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Dec 13, 2024 at 12:03:42AM +0100, Christian Brauner wrote:
> +static inline void mnt_ns_tree_write_lock(void)
> +{
> +	write_lock(&mnt_ns_tree_lock);
> +	write_seqcount_begin(&mnt_ns_tree_seqcount);
> +}
> +
> +static inline void mnt_ns_tree_write_unlock(void)
> +{
> +	write_seqcount_end(&mnt_ns_tree_seqcount);
> +	write_unlock(&mnt_ns_tree_lock);
>  }

>  static void mnt_ns_tree_add(struct mnt_namespace *ns)
>  {
> -	guard(write_lock)(&mnt_ns_tree_lock);
> -	rb_add(&ns->mnt_ns_tree_node, &mnt_ns_tree, mnt_ns_less);
> +	struct rb_node *node;
> +
> +	mnt_ns_tree_write_lock();
> +	node = rb_find_add_rcu(&ns->mnt_ns_tree_node, &mnt_ns_tree, mnt_ns_cmp);
> +	mnt_ns_tree_write_unlock();
> +
> +	WARN_ON_ONCE(node);
>  }

>  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);
>  }

Its probably not worth the effort, but I figured I'd mention it anyway,
if you do:

DEFINE_LOCK_GUARD_0(mnt_ns_tree_lock, mnt_ns_tree_lock(), mnt_ns_tree_unlock())

You can use: guard(mnt_ns_tree_lock)();

But like said, probably not worth it, given the above are the only two
sites and they're utterly trivial.


Anyway, rest of the patches look good now.