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

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Date:	Mon, 17 Nov 2014 12:24:08 -0800
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Alexander Duyck <alexander.h.duyck@...hat.com>
CC:	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 11/17/2014 12:04 PM, Benjamin Herrenschmidt wrote:
> On Mon, 2014-11-17 at 09:18 -0800, Alexander Duyck wrote:
>> There are a number of situations where the mandatory barriers rmb() and
>> wmb() are used to order memory/memory operations in the device drivers
>> and those barriers are much heavier than they actually need to be.  For
>> example in the case of PowerPC wmb() calls the heavy-weight sync
>> instruction when for memory/memory operations all that is really needed is
>> an lsync or eieio instruction.
> So essentially those are the same as the smp_* variants but not nop'ed
> out on !CONFIG_SMP right ? 
>
> Ben.
>

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.

Thanks,

Alex
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