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

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Message-ID: <AANLkTi=jz+mGru--+YA62EeER17Tuq2DocrkKNaU2k+1@mail.gmail.com>
Date:	Mon, 14 Feb 2011 17:50:04 +0000
From:	Will Newton <will.newton@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Jason Baron <jbaron@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	hpa@...or.com, 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, davem@...emloft.net, sam@...nborg.org,
	ddaney@...iumnetworks.com, michael@...erman.id.au,
	linux-kernel@...r.kernel.org, Mike Frysinger <vapier@...too.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	dhowells <dhowells@...hat.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	"heiko.carstens" <heiko.carstens@...ibm.com>,
	benh <benh@...nel.crashing.org>
Subject: Re: [PATCH 0/2] jump label: 2.6.38 updates

On Mon, Feb 14, 2011 at 5:43 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, 2011-02-14 at 17:38 +0000, Will Newton wrote:
>> On Mon, Feb 14, 2011 at 5:27 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>>
>> >> So all but a few have basically (as you said on IRC)
>> >> #define atomic_read(v) ACCESS_ONCE(v)
>> >
>> > ACCESS_ONCE(v->counter), but yeah :-)
>>
>> I maintain an out-of-tree architecture where that isn't the case
>> unfortunately [1]. Not expecting any special favours for being
>> out-of-tree of course, but just thought I would add that data point.
>>
>> [1] Our atomic operations go around the cache rather than through it,
>> so the value of an atomic cannot be read with a normal load
>> instruction.
>
> Cannot how? It would observe a stale value? That is acceptable for
> atomic_read().

It would observe a stale value, but that value would only be updated
when the cache line was reloaded from main memory which would have to
be triggered by either eviction or cache flushing. So it could get
pretty stale. Whilst that's probably within the spec. of atomic_read I
suspect it would lead to problems in practice. I could be wrong
though.
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