[PATCH, libstdc++] Use acquire semantics in case of CAS failure

[Thiago Macieira]

Hello

I detected this issue as I was updating the patches to send to the mailing 
list. I have not created a bug report.

When the CAS operation fails and expected == guard_bit, __cxa_guard_acquire 
will return immediately indicating that the initialisation has already 
succeeded. However, it's missing the acquire barrier for the changes done on 
the other thread, to match the release barrier from __cxa_guard_release.

That is:
thread A					thread B
load.acq == 0				load.acq == 0
__cxa_guard_acquire			__cxa_guard_acquire
CAS(0 -> 256) success
__cxa_guard_release
store.rel(1)
							CAS(0 ->256) fails

At this point, we must synchronise with the store-release from thread A.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center
     Intel Sweden AB - Registration Number: 556189-6027
     Knarrarnäsgatan 15, 164 40 Kista, Stockholm, Sweden
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