Re: [RFC PATCH 2/2] RDMA/rxe: Add RDMA Atomic Write operation

["yangx.jy@fujitsu.com" <yangx.jy@fujitsu.com>]

On 2022/1/6 21:00, Jason Gunthorpe wrote:
> On Thu, Jan 06, 2022 at 10:52:47AM +0000, yangx.jy@fujitsu.com wrote:
>> On 2022/1/6 7:53, Jason Gunthorpe wrote:
>>> On Thu, Dec 30, 2021 at 04:39:01PM -0500, Tom Talpey wrote:
>>>
>>>> Because RXE is a software provider, I believe the most natural approach
>>>> here is to use an atomic64_set(dst, *src).
>>> A smp_store_release() is most likely sufficient.
>> Hi Jason, Tom
>>
>> Is smp_store_mb() better here? It calls WRITE_ONCE + smb_mb/barrier().
>> I think the semantics of 'atomic write' is to do atomic write and make
>> the 8-byte data reach the memory.
> No, it is not 'data reach memory' it is a 'release' in that if the CPU
> later does an 'acquire' on the written data it is guarenteed to see
> all the preceeding writes.
Hi Jason, Tom

Sorry for the wrong statement. I mean that the semantics of 'atomic 
write' is to write an 8-byte value atomically and make the 8-byte value 
visible for all CPUs.
'smp_store_release' makes all the preceding writes visible for all CPUs 
before doing an atomic write. I think this guarantee should be done by 
the preceding 'flush'.
'smp_store_mb' does an atomic write and then makes the atomic write 
visible for all CPUs. Subsequent 'flush' is only used to make the atomic 
write persistent.
Please correct me if my understand is wrong.

Best Regards,
Xiao Yang
> Memory barriers are always paired with read and write, it is important
> to understand what the read half of the pair is.
>
> Jason