Re: [PATCH] nvme: fix namespace removal list

[Christoph Hellwig <hch@lst.de>]

On Tue, Jun 11, 2024 at 11:22:04AM -0600, Keith Busch wrote:
> > Is this actually valid for a (S)RCU protected list?  If the entry gets
> > added to the new list before the grace period has completed, we could
> > trick a concurrent traversal into following the new list unless I'm
> > mistaken (although chances I'm mistaken on RCU corner cases aren't that
> > low..).
> 
> Good call, you are absolutely right that a sync should happen between
> the del and the add for readers to consistently iterate this list.
> 
> I might be able to weasel out of this though: our namespace list is
> sorted, and this function wants to append everything from this element
> all the way to the end to the "rm_list": a reader should get the same
> result either way, whether if it was torn at this element or the move
> happened without the reader seeing it.
> 
> Now if there was a way to rcu split the list so we don't have to do it
> element by element...

Isn't that exactly what the list_splice helpers do?  We'd just need to
do that exactly once at the cutoff point and not once per namespace.