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Don Bowman writes:
 > 
 > On the P4, there are mfence,lfence,sfence instructions to enforce
 > memory ordering. These are cheaper than "lock; andl" or "cpuid",
 > which are the traditional 'sync' instructions.

For what its worth, using those operations yeilds these results
on my 2.53GHz P4 (for UP)

Mutex (atomic_store_rel_int) cycles per iteration: 208 
Mutex (sfence) cycles per iteration: 85 
Mutex (lfence) cycles per iteration: 63 
Mutex (mfence) cycles per iteration: 169 
Mutex (none) cycles per iteration: 18 

lfence looks like a winner..

Drew