On 05/19/2016 11:05 AM, Jason Gunthorpe wrote:
This is the mapping that matches the definitions of the barriers: wmb can be memory_order_release rmb can be memory_order_acquire mb can be memory_order_seq_cst
This is not correct. Acquire and release, at least as defined by Gharachorloo in 1991, have another meaning than the mentioned barrier instructions. Acquire and release are labels that can be applied to load and store instructions respectively. Acquire semantics prevent reordering of a load-acquire instruction with any read or write operation which follows it in program order. Release semantics prevent reordering of a store-release with any read or write operation which precedes it in program order. Figure 2 in https://courses.engr.illinois.edu/cs533/sp2012/reading_list/2b.pdf illustrates this clearly.
I have not yet analyzed the meaning of acquire and release labels in the Itanium instruction set nor in the C11 memory consistency model closely but I think that acquire and release have the same meaning in these contexts as what has been explained above.
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