Memory barrier for fat initialization

[Niels Möller]

GMP's fat initialization (I'm looking at the x86_64 code now) ends with

  *((volatile int *) &__gmpn_cpuvec_initialized) = 1;

I suspect that it's possible (but unlikely) that a different thread on
another cpu may read __gmpn_cpuvec_initialized, get 1, read thresholds
or pointers, and still get old values. I'm about to add fat support to
the nettle library, and to address this, I think I'll replace the
corresponding assignment with

  _nettle_synchronous_write (&initialized, 1);

I implement this in the same file as the cpuid function, as

PROLOGUE(_nettle_synchronous_write)
	W64_ENTRY(2)
	mfence
	movl	%esi, (%rdi)
	mfence
	W64_EXIT(2)
        ret
EPILOGUE(_nettle_synchronous_write)

Does that make sense? I'm not very familiar with these memory model
issues. I don't think the second mfence really is necessary, but it
makes some sense to me to push the new value out to the other cpus as
soon as it's written.

Regards,
/Niels

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