port-vax/55094: vax pmap needs locking adjustments

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>Number:         55094
>Category:       port-vax
>Synopsis:       vax pmap needs locking adjustments
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-vax-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Mar 21 17:10:00 +0000 2020
>Originator:     Andrew Doran
>Release:        NetBSD 9.99.50
>Organization:
	The NetBSD Project
>Environment:
n/a
>Description:
	The vax pmap looks like it needs locking at the pmap level, to
	protect against multi-threaded programs doing concurrent operations.
	It probably works OK for pmap_kernel since there is PV list locking,
	and you don't tend to get concurrent modifications to the same VA in
	pmap_kernel.
>How-To-Repeat:
	Code inspection.
>Fix:
	Have a global pmap_lock.  That would allow removal of the atomics
	adjusting stats, and removal of pvtable_lock.  Low maintainence
	overhead and low runtime cost.