There are three problems with x86 membar:

1) It's a no-op on win32

2) Linux uses sfence, instead of mfence, doing only store barriers

3) Solaris x86 claims to do a mfence, but uses the sfence encoding!

This is a critical flaw. There is a reproducable VM crash on win32,
and the solx86/linux bug could easily result in random VM crashes.

Never use a machine with more than one cpu.

Crash is reproducable on win32. Its a bug.

There is some risk that the fix will have an effect on performance, but
the only other choice is to give the wrong answer or crash. 

  xxxxx@xxxxx   2003-03-04

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Fix for mantis:

1. change i486 assembler to emit the correct opcode
2. Win32 and Linux atomic::membar() use locked-add as memory barriers.

  xxxxx@xxxxx   2003-03-07


Fix for 1.4.1_06:
1. change i486 assembler to emit the correct opcode
2. Win32 atomic::membar() use locked-add as memory barriers, for linux it is not applicable.

  xxxxx@xxxxx   2003-09-16