[PATCH, libstdc++] Fix PR54172

[Thiago Macieira]

Hello

The attached patch fixes a race condition in __cxa_guard_acquire, a regression 
from 4.6. When the code was refactored to use the new atomic intrinsics, the 
fact that __atomic_compare_exchange_n updates the "expected" variable with the 
current value was missed.

That causes the loop to possibly overwrite a successful initialisation and re-
start the initialisation phase. The bug report has a gdb trace with a 
watchpoint showing the guard variable transitioning

	0 (uninit) -> 256 (pending) -> 1 (done) -> 256 (pending) -> 1 (done)

This situation can happen if both CAS fail in given thread: the first because 
the other thread started the initialisation and the second because the 
initialisation succeeded.

-- 
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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