[RTEMS Project] #3175: Merge FreeBSD timecounter changes from 2015-01-20 to now

[RTEMS trac]

#3175: Merge FreeBSD timecounter changes from 2015-01-20 to now
-----------------------------+------------------------------
 Reporter:  Sebastian Huber  |       Owner:  Sebastian Huber
     Type:  enhancement      |      Status:  assigned
 Priority:  normal           |   Milestone:  4.12.0
Component:  score            |     Version:  4.12
 Severity:  normal           |  Resolution:
 Keywords:                   |
-----------------------------+------------------------------

Comment (by Konstantin Belousov <kib@…>):

 In [changeset:"016306354f8d68c894a3374471f28b11f0226bd4/rtems"
 0163063/rtems]:
 {{{
 #!CommitTicketReference repository="rtems"
 revision="016306354f8d68c894a3374471f28b11f0226bd4"
 timecounter: Merge FreeBSD change r285286

 Reimplement the ordering requirements for the timehands updates, and for
 timehands consumers, by using fences.

 Ensure that the timehands->th_generation reset to zero is visible
 before the data update is visible [*].  tc_setget() allowed data update
 writes to become visible before generation (but not on TSO
 architectures).

 Remove tc_setgen(), tc_getgen() helpers, use atomics inline [**].

 Noted by:       alc [*]
 Requested by:   bde [**]
 Reviewed by:    alc, bde
 Sponsored by:   The FreeBSD Foundation
 MFC after:      3 weeks

 Update #3175.
 }}}

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Ticket URL: <http://devel.rtems.org/ticket/3175#comment:5>
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