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

[Jason Gunthorpe]

On Tue, Jun 18, 2019 at 06:27:22AM -0700, Christoph Hellwig wrote:
> 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.

With the previous loose coupling of the mirror and the range some code
might rance to try to create a range without a mirror, which will now
reliably crash with the poison.

It isn't so much the double unregister that worries me, but racing
unregister with range functions.

Jason