linux-kernel - Re: LKMM: Read dependencies of writes ordered by dma

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Message-ID: <20210817122816.GA12746@willie-the-truck>
Date:   Tue, 17 Aug 2021 13:28:16 +0100
From:   Will Deacon <will@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Marco Elver <elver@...gle.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
        linux-kernel@...r.kernel.org
Subject: Re: LKMM: Read dependencies of writes ordered by dma_wmb()?

Just on this bit...

On Mon, Aug 16, 2021 at 01:50:57PM -0700, Paul E. McKenney wrote:
> 5.	The dma_mb(), dma_rmb(), and dma_wmb() appear to be specific
> 	to ARMv8.

These are useful on other architectures too! IIRC, they were added by x86 in
the first place. They're designed to be used with dma_alloc_coherent()
allocations where you're sharing something like a ring buffer with a device
and they guarantee accesses won't be reordered before they become visible
to the device. They _also_ provide the same ordering to other CPUs.

I gave a talk at LPC about some of this, which might help (or might make
things worse...):

https://www.youtube.com/watch?v=i6DayghhA8Q

Ignore the bits about mmiowb() as we got rid of that.

Will