Excerpts from Nicholas Piggin's message of July 13, 2020 2:45 pm:

Excerpts from Andy Lutomirski's message of July 11, 2020 3:04 am:

quoted

Also, as it stands, I can easily see in_irq() ceasing to promise to
serialize.  There are older kernels for which it does not promise to
serialize.  And I have plans to make it stop serializing in the
nearish future.

You mean x86's return from interrupt? Sounds fun... you'll konw where to 
update the membarrier sync code, at least :)

Oh, I should actually say Mathieu recently clarified a return from
interrupt doesn't fundamentally need to serialize in order to support
membarrier sync core.

https://lists.ozlabs.org/pipermail/linuxppc-dev/2020-July/214171.html

So you may not need to do anything more if you relaxed it.

Thanks,
Nick