Re: [PATCH v2 1/2] srcu: Add list_splice_init_srcu()

[Qiliang Yuan <odys.yuan@gmail.com>]

On Wed, Nov 04, 2026 at 09:14:12AM -0800, Paul E. McKenney wrote:
> I am not inalterably opposed to adding a list_splice_init_srcu(),
> especially if quite a few more users show up, but here is what I was
> expecting people to do:
>
> 	static void synchronize_my_srcu(void)
> 	{
> 		synchronize_srcu(&my_srcu);
> 	}
>
> 	...
>
> 	list_splice_init_rcu(l, h, synchronize_my_srcu);
>
> We need at least six uses of the above idiom to break even on the line
> of code required for introducing a list_splice_init_srcu().

That works for a global srcu_struct, but nvme has one per controller,
next to the list it protects:

	struct nvme_ctrl {
		...
		struct list_head namespaces;
		struct mutex namespaces_lock;
		struct srcu_struct srcu;
		...
	};

nvme_init_ctrl() calls init_srcu_struct(&ctrl->srcu) for every
controller, and readers walk the list under that controller's srcu,
e.g. nvme_wait_freeze():

	srcu_idx = srcu_read_lock(&ctrl->srcu);
	list_for_each_entry_srcu(ns, &ctrl->namespaces, list,
				 srcu_read_lock_held(&ctrl->srcu))
		blk_mq_freeze_queue_wait(ns->queue);
	srcu_read_unlock(&ctrl->srcu, srcu_idx);

nvme_remove_namespaces() thus has to wait on the srcu of the controller
being deleted, which a sync function that takes no argument can't get
at.

So the choice here is between list_splice_init_srcu(), as Christoph
suggested on v1, and open-coding the splice in nvme_remove_namespaces()
as v1 did. Which would you prefer?

Thanks,
Qiliang