linux-kernel - Re: [PATCH 0/2] jump label: 2.6.38 updates

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Date:	Tue, 15 Feb 2011 11:01:21 +0000
From:	Will Newton <will.newton@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Matt Fleming <matt@...sole-pimps.org>,
	David Miller <davem@...emloft.net>, rostedt@...dmis.org,
	peterz@...radead.org, jbaron@...hat.com,
	mathieu.desnoyers@...ymtl.ca, mingo@...e.hu, tglx@...utronix.de,
	andi@...stfloor.org, roland@...hat.com, rth@...hat.com,
	masami.hiramatsu.pt@...achi.com, fweisbec@...il.com,
	avi@...hat.com, sam@...nborg.org, ddaney@...iumnetworks.com,
	michael@...erman.id.au, linux-kernel@...r.kernel.org,
	vapier@...too.org, cmetcalf@...era.com, dhowells@...hat.com,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
	benh@...nel.crashing.org
Subject: Re: [PATCH 0/2] jump label: 2.6.38 updates

On Mon, Feb 14, 2011 at 11:19 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 02/14/2011 02:37 PM, Matt Fleming wrote:
>>>
>>> I don't see how cache coherency can possibly work if the hardware
>>> behaves this way.
>>
>> Cache coherency is still maintained provided writes/reads both go
>> through the cache ;-)
>>
>> The problem is that for read-modify-write operations the arbitration
>> logic that decides who "wins" and is allowed to actually perform the
>> write, assuming two or more CPUs are competing for a single memory
>> address, is not implemented in the cache controller, I think. I'm not a
>> hardware engineer and I never understood how the arbitration logic
>> worked but I'm guessing that's the reason that the ll/sc instructions
>> bypass the cache.
>>
>> Which is why the atomic_t functions worked out really well for that
>> arch, such that any accesses to an atomic_t * had to go through the
>> wrapper functions.
>
> I'm sorry... this doesn't compute.  Either reads can work normally (go
> through the cache) in which case atomic_read() can simply be a read or
> they don't, so I don't understand this at all.

The CPU in question has two sets of instructions:

  load/store - these go via the cache (write through)
  ll/sc - these operate literally as if there is no cache (they do not
hit on read or write)

This may or may not be a sensible way to architect a CPU, but I think
it is possible to make it work. Making it work efficiently is more of
a challenge.
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