Complex sentence: On most non-x86 architectures, explicit memory barrier instructions or atomic instructions (as in the example) must be used, or there may be special "unlock" instructions (as on IA-64) which provide the needed memory ordering.
Split into simpler sentences: On most non-x86 architectures, explicit memory barrier or atomic instructions (as in the example) must be used. On some systems, such as IA-64, there are special "unlock" instructions which provide the needed memory ordering.