introduce sys_membarrier(): process-wide memory barrier (v8)

Hi,

Here is an updated version of the sys_membarrier system call. It implements a
system call that allows userspace to distribute the overhead of memory barriers
asymmetrically. It is particularly useful for Userspace RCU (liburcu), to allow
the RCU read-side to only issue compiler barriers, matched by calls to
sys_membarrier() in the userspace synchronize_rcu().

Even though sys_membarrier should be portable to other architecture as-is
without arch-specific modifications, I propose to incrementally reserve the
system call IDs as we test it on each architecture.

Here is a version that I think should be ready for merging. It is based on the
2.6.33-rc6 kernel. I would appreciate if people who gave their Acked-by in the
RFC rounds could confirm their Acked-by following the patch set updates.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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