linux-kernel - Re: Re: [PATCH] rdma/siw: avoid smp_store

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Date:   Fri, 12 Jul 2019 15:22:35 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Bernard Metzler <BMT@...ich.ibm.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Doug Ledford <dledford@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Re: [PATCH] rdma/siw: avoid smp_store_mb() on a u64

On Fri, Jul 12, 2019 at 3:05 PM Bernard Metzler <BMT@...ich.ibm.com> wrote:

>
> We share CQ (completion queue) notification flags between application
> (which may be user land) and producer (kernel QP's (queue pairs)).
> Those flags can be written by both application and QP's. The application
> writes those flags to let the driver know if it shall inform about new
> work completions. It can write those flags at any time.
> Only a kernel producer reads those flags to decide if
> the CQ notification handler shall be kicked, if a new CQ element gets
> added to the CQ. When kicking the completion handler, the driver resets the
> notification flag, which must get re-armed by the application.
>
> We use READ_ONCE() and WRITE_ONCE(), since the flags are potentially
> shared (mmap'd) between user and kernel land.
>
> siw_req_notify_cq() is being called only by kernel consumers to change
> (write) the CQ notification state. We use smp_store_mb() to make sure
> the new value becomes visible to all kernel producers (QP's) asap.
>
>
> From cfb861a09dcfb24a98ba0f1e26bdaa1529d1b006 Mon Sep 17 00:00:00 2001
> From: Bernard Metzler <bmt@...ich.ibm.com>
> Date: Fri, 12 Jul 2019 13:19:27 +0200
> Subject: [PATCH] Make shared CQ notification flags 32bit to respect 32bit
>  architectures
>
> Signed-off-by: Bernard Metzler <bmt@...ich.ibm.com>

This fixes the build for me, thanks!

Tested-by: Arnd Bergmann <arnd@...db.de>