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

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Message-ID: <1416344873.5704.12.camel@kernel.crashing.org>
Date:	Wed, 19 Nov 2014 08:07:53 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Alexander Duyck <alexander.h.duyck@...hat.com>
Cc:	Alexander Duyck <alexander.duyck@...il.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 19:13 -0800, Alexander Duyck wrote:
> 
> ARM adds some funky things.  They have two different types of 
> primitives, a dmb() which is a data memory barrier, and a dsb() which is 
> a data synchronization barrier.  Then with each of those they have the 
> "domains" the barriers are effective within.
> 
> So for example on ARM a rmb() is dsb(sy) which means it is a system wide 
> synchronization barrier which stops execution on the CPU core until the 
> read completes.  

That's amazingly heavy handed ... I can see that being useful for MMIO,
we do something similar in our MMIO accessors by using a special variant
of trap instruction that never traps to make the core thing the load
value has been consumed. But that's typically only needed to guarantee
MMIO timings.

> However the smp_rmb() is a dmb(ish) which means it is 
> only a barrier as far as the inner shareable domain which I believe only 
> goes as far as the local shared cache hierarchy and only guarantees read 
> ordering without necessarily halting the CPU or stopping in-order 
> speculative reads.  So what a coherent_rmb() would be in my setup is 
> dmb(sy) which means the barrier runs all the way out to memory, and it 
> is allowed to speculative read as long as it does it in order.

Correct, which is thus the same as smp_rmb() ... which was my original
point, or am I missing something else ?

> If it is still unclear you might check out Will Deacon's talk on the 
> topic at https://www.youtube.com/watch?v=6ORn6_35kKo, at about 7:00 in 
> he explains the whole domains thing, and at 13:30 he explains dmb()/dsb().

Ok, I'll try to watch that when I get a chance.

Cheers,
Ben.


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