Re: [PATCH v4 1/6] RDMA/bnxt_re: Eliminate duplicate barriers on weakly-ordered archs

[Jason Gunthorpe]

On Tue, Mar 20, 2018 at 10:08:16AM -0500, Sinan Kaya wrote:

> Never mind, it will break some architectures. I'll only change the first one.
> 
>  (1) On some systems, I/O stores are not strongly ordered across all CPUs, and
>      so for _all_ general drivers locks should be used and mmiowb() must be
>      issued prior to unlocking the critical section.

I think the kernel could do well to have a spin_unlock_mmiowb()
function. We have this patern quite a bit.

Arches like x86 can just make it == spin_unlock, while PPC and ARM can
add their extra barriers.

Then we can safely and efficiently use _realxed within such a
spinlock region.

Jason