linux-kernel - Re: [PATCH v4 0/4] Add lightweight memory barriers for coherent memory access

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Message-ID: <CA+55aFw3ygP8UEF1vidWdjBUEkqXOELCPkpjLMD-X=vXXQEGOQ@mail.gmail.com>
Date:	Tue, 18 Nov 2014 12:53:34 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Alexander Duyck <alexander.h.duyck@...hat.com>
Cc:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Peter Zijlstra <peterz@...radead.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Michael Neuling <mikey@...ling.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	donald.c.skidmore@...el.com, matthew.vick@...el.com,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Francois Romieu <romieu@...zoreil.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	nic_swsd@...ltek.com, Will Deacon <will.deacon@....com>,
	Michael Ellerman <michael@...erman.id.au>,
	Tony Luck <tony.luck@...el.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH v4 0/4] Add lightweight memory barriers for coherent
 memory access

On Tue, Nov 18, 2014 at 9:28 AM, Alexander Duyck
<alexander.h.duyck@...hat.com> wrote:
> These patches introduce two new primitives for synchronizing cache coherent
> memory writes and reads.  These two new primitives are:
>
>         coherent_rmb()
>         coherent_wmb()

So I'm still not convinced about the name. I don't hate it, but if you
ever want to do "read_acquire", then that whole "coherent_" thing does
make for a big mouthful. I don't see why "dma" isn't simpler and more
to the point, and has the advantage of lining up (in documentation
etc) with "smp".

Why would you ever use "coherent_xyz()" on something that isn't about
dma? If it's cache-coherent memory without DMA, you'd use "smp_xyz()",
so I really do prefer that whole "dma-vs-smp" issue, because it talks
about what is actually the important issue. All sane memory is
coherent, after all (and if it isn't, you have other issues than
memory ordering).

                  Linus
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