Re: [PATCH nf,v2 1/3] rculist: add list_splice_rcu() for private lists

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Wed, Apr 15, 2026 at 10:39:33AM -0700, Paul E. McKenney wrote:
> On Wed, Apr 15, 2026 at 07:08:44PM +0200, Pablo Neira Ayuso wrote:
> > This patch adds a helper function, list_splice_rcu(), to safely splice
> > a private (non-RCU-protected) list into an RCU-protected list.
> > 
> > The function ensures that only the pointer visible to RCU readers
> > (prev->next) is updated using rcu_assign_pointer(), while the rest of
> > the list manipulations are performed with regular assignments, as the
> > source list is private and not visible to concurrent RCU readers.
> > 
> > This is useful for moving elements from a private list into a global
> > RCU-protected list, ensuring safe publication for RCU readers.
> > Subsystems with some sort of batching mechanism from userspace can
> > benefit from this new function.
> > 
> > The function __list_splice_rcu() has been added for clarity and to
> > follow the same pattern as in the existing list_splice*() interfaces,
> > where there is a check to ensure that that the list to splice is not
> > empty. Note that __list_splice_rcu() has no documentation for this
> > reason.
> > 
> > Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
> > ---
> > v2: including comments by Paul McKenney.
> > 
> >     Except, I have deliberately keep back the suggestion to squash
> >     __list_splice_rcu() into list_splice_rcu(), I instead removed
> >     the documentation for __list_splice_rcu(). I am looking
> >     at other existing list_splice*() function in list.h and rculist.h
> >     to get this aligned with __list_splice(), which also has no users
> >     in the tree and no documentation. I find it easier to read with
> >     __list_splice(), but if this explaination is not sound so...
> > 
> >     @Paul: I can post v3 squashing __list_splice_rcu(), just let me
> >            know.
> 
> Removing the comment addresses most of my concerns.  I do have a slight
> but not overwhelming preference for the squashed version, but either way:
> 
> Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
> 
> Or if you want this to go in via RCU, please let us know.  My guess is
> that it would be easier for you to take it in with the code using it.

I'd prefer to take it through nf.git, I need this as a fix for an
invalid use of list_splice() on a RCU-protected list.

Thanks for your quick review Paul!