[PATCH] nvme: fix namespace removal list

[Keith Busch]

On Tue, Jun 11, 2024 at 07:24:48PM +0200, Christoph Hellwig wrote:
> 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.

Not a splice since that takes the entire source list; this usage is more
aligned to "list_cut_position()", which doesn't look rcu safe as-is, but
I think we can make one!