[RFR]: 8187590: Zero runtime can lock-up on linux-alpha

[John Paul Adrian Glaubitz]

Hi!

Please review this change [1] which fixes random lockups of the Zero runtime
on linux-alpha. This was discovered when building OpenJDK on the Debian
automatic package builders for Alpha, particularly on SMP machines.

It was observed that the issue could be fixed by installing a uni-processor
kernel. After some testing, we discovered that the proper fix is to use
__sync_synchronize() even for light memory barriers.

Thanks,
Adrian

> [1] http://cr.openjdk.java.net/~glaubitz/8187590/webrev.02/

-- 
 .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer - glaubitz at debian.org
`. `'   Freie Universitaet Berlin - glaubitz at physik.fu-berlin.de
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