K 10
svn:author
V 3
mjg
K 8
svn:date
V 27
2017-01-03T21:36:15.361964Z
K 7
svn:log
V 455
mtx: reduce lock accesses

Instead of spuriously re-reading the lock value, read it once.

This change also has a side effect of fixing a performance bug:
on failed _mtx_obtain_lock, it was possible that re-read would find
the lock is unowned, but in this case the primitive would make a trip
through turnstile code.

This is diff reduction to a variant which uses atomic_fcmpset.

Discussed with:	jhb (previous version)
Tested by:	pho (previous version)

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