linux-kernel - Re: [PATCH 2/4] arch: Add lightweight memory barriers fast_rmb() and fast

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Date:	Tue, 18 Nov 2014 11:39:27 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Alexander Duyck <alexander.h.duyck@...hat.com>,
	linux-arch@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, mathieu.desnoyers@...ymtl.ca,
	peterz@...radead.org, heiko.carstens@...ibm.com, mingo@...nel.org,
	mikey@...ling.org, linux@....linux.org.uk,
	donald.c.skidmore@...el.com, matthew.vick@...el.com,
	geert@...ux-m68k.org, jeffrey.t.kirsher@...el.com,
	romieu@...zoreil.com, paulmck@...ux.vnet.ibm.com,
	nic_swsd@...ltek.com, will.deacon@....com, michael@...erman.id.au,
	tony.luck@...el.com, torvalds@...ux-foundation.org,
	oleg@...hat.com, schwidefsky@...ibm.com, fweisbec@...il.com,
	davem@...emloft.net
Subject: Re: [PATCH 2/4] arch: Add lightweight memory barriers fast_rmb()
 and fast_wmb()

On Mon, 2014-11-17 at 12:24 -0800, Alexander Duyck wrote:
> Yes and no.  So for example on ARM I used the dmb() operation, however
> I
> have to use the barrier at the system level instead of just the inner
> shared domain.  However on many other architectures they are just the
> same as the smp_* variants.
> 
> Basically the resultant code is somewhere between the smp and non-smp
> barriers in terms of what they cover.

There I don't quite follow you. You need to explain better especially in
the documentation because otherwise people will get it wrong...

If it's ordering in the coherent domain, I fail to see how a DMA agent
is different than another processor when it comes to barriers, so I fail
to see the difference with smp_*

I understand the MMIO vs. memory issue, we do have the same on powerpc,
but that other aspect eludes me.

Ben.


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