gnttab: split maptrack lock to make it fulfill its purpose again

gnttab: split maptrack lock to make it fulfill its purpose again

The way the lock is currently being used in get_maptrack_handle(), it
protects only the maptrack limit: The function acts on current's list
only, so races on list accesses are impossible even without the lock.

Otoh list access races are possible between __get_maptrack_handle() and
put_maptrack_handle(), due to the invocation of the former for other
than current from steal_maptrack_handle(). Introduce a per-vCPU lock
for list accesses to become race free again. This lock will be
uncontended except when it becomes necessary to take the steal path,
i.e. in the common case there should be no meaningful performance
impact.

When in get_maptrack_handle adds a stolen entry to a fresh, empty,
freelist, we think that there is probably no concurrency.  However,
this is not a fast path and adding the locking there makes the code
clearly correct.

Also, while we are here: the stolen maptrack_entry's tail pointer was
not properly set.  Set it.

This is XSA-228.

Reported-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Gbp-Pq: Name gnttab-split-maptrack-lock-to-make-it-fu