[PATCH v3 hmm 08/12] mm/hmm: Remove racy protection against double-unregistration

[Christoph Hellwig]

On Tue, Jun 18, 2019 at 10:13:24AM -0300, Jason Gunthorpe wrote:
> > I don't even think we even need to bother with the POISON, normal list
> > debugging will already catch a double unregistration anyway.
> 
> mirror->hmm isn't a list so list debugging won't help.
> 
> My concern when I wrote this was that one of the in flight patches I
> can't see might be depending on this double-unregister-is-safe
> behavior, so I wanted them to crash reliably.
> 
> It is a really overly conservative thing to do..

mirror->list is a list, and if we do a list_del on it during the
second unregistration it will trip up on the list poisoning.