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

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Message-ID: <AANLkTinzr6rb=WwFs7QApsvdy5f7PHZ1qS9ZVrncEzZD@mail.gmail.com>
Date:	Wed, 16 Feb 2011 12:41:59 +0000
From:	Will Newton <will.newton@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Will Simoneau <simoneau@....uri.edu>,
	David Miller <davem@...emloft.net>, hpa@...or.com,
	matt@...sole-pimps.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 Wed, Feb 16, 2011 at 12:18 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Wed, 2011-02-16 at 10:15 +0000, Will Newton wrote:
>
>> > That's some really crippled hardware... it does seem like *any* loads
>> > from *any* address updated by an sc would have to be done with ll as
>> > well, else they may load stale values. One could work this into
>> > atomic_read(), but surely there are other places that are problems.
>>
>> I think it's actually ok, atomics have arch implemented accessors, as
>> do spinlocks and atomic bitops. Those are the only place we do sc so
>> we can make sure we always ll or invalidate manually.
>
> I'm curious, how is cmpxchg() implemented on this architecture? As there
> are several places in the kernel that uses this on regular variables
> without any "accessor" functions.

We can invalidate the cache manually. The current cpu will see the new
value (post-cache invalidate) and the other cpus will see either the
old value or the new value depending on whether they read before or
after the invalidate, which is racy but I don't think it is
problematic. Unless I'm missing something...
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