linux-kernel - Re: [RFC][PATCH 0/5] Introduce checks for preemptable code for this_cpu

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Date:	Tue, 20 Sep 2011 15:51:32 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Valdis.Kletnieks@...edu
cc:	Steven Rostedt <rostedt@...dmis.org>,
	Christoph Lameter <cl@...two.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [RFC][PATCH 0/5] Introduce checks for preemptable code for
 this_cpu_read/write()

On Tue, 20 Sep 2011, Valdis.Kletnieks@...edu wrote:

> On Mon, 19 Sep 2011 23:06:17 EDT, Steven Rostedt said:
> 
> > It is really confusing to know which version to use. I'm confused by the
> > this_cpu_*() compared with __this_cpu_*(). I'm guessing that most places
> > should use __this_cpu*(). But really this_cpu() should be the default,
> > and the places that can have it outside of preemption should have
> > another name. Maybe use the raw_this_cpu() or safe_this_cpu(), as there
> > is an irqsafe_this_cpu(). Maybe make a preemptsafe_cpu_*(). There should
> > only be a very few locations that are OK to have preemption enabled when
> > calling the this_cpu() code. Lets have those have the funny names and
> > not be the default "this_cpu_*()".
> 
> What's the latency hit on those very few locations if we simply put our
> collective foot down and not support a preemptable version of this_cpu_*()?
> "Yes, you *could* preempt here, but for our collective sanity that's not
> supported"...

Full ack.
 
> > All this_cpu*() code, except the funny named ones, should make sure
> > preemption is disabled, otherwise give a nasty warning. As that is
> > usually a bug if you are using a per cpu variable and can migrate away.
> > The next reference to that value may be incorrect.
> 
> You get a much prettier diffstat if you just nuke the funny named ones. ;)

Along with the maze of completely unused incarnations.
 
> But of course it's early morning and I'm still caffeine-deficient and probably
> overlooking some crucial use case. ;)

I doubt that.

Thanks,

	tglx
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