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

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Date:	Mon, 14 Feb 2011 19:04:27 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Newton <will.newton@...il.com>
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, 2011-02-14 at 17:50 +0000, Will Newton wrote:
> 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.

Arguable, finding such cases would be a Good (TM) thing.. but yeah, I
can imagine you're not too keen on being the one finding them.

Luckily it looks like you're in the same boat as blackfin-smp is.

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