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Date: Tue, 20 Mar 2018 09:23:59 -0600
From: Jason Gunthorpe <jgg@ziepe.ca>
To: Sinan Kaya <okaya@codeaurora.org>
Cc: linux-rdma@vger.kernel.org, timur@codeaurora.org,
        sulrich@codeaurora.org, linux-arm-msm@vger.kernel.org,
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        Sriharsha Basavapatna <sriharsha.basavapatna@broadcom.com>,
        Doug Ledford <dledford@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 1/6] RDMA/bnxt_re: Eliminate duplicate barriers on
 weakly-ordered archs
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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