linux-kernel - Re: [PATCH] preempt/hardirq.h: Clarify PREEMPT

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Date:	Tue, 14 Aug 2012 08:46:34 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	mingo@...nel.org, paulmck@...ux.vnet.ibm.com,
	josh@...htriplett.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] preempt/hardirq.h: Clarify PREEMPT_ACTIVE bit location

On Tue, 2012-08-14 at 12:18 +0200, Thomas Gleixner wrote:
> On Tue, 14 Aug 2012, Srivatsa S. Bhat wrote:
> 
> > 
> > By default, the PREEMPT_ACTIVE flag is bit 27, but different
> > architectures can override that. Update the comment to reflect
> > this fact.
> 
> Is there any sensible reason why architectures need to override that?
> I can't find one.
> 
> If there is none, then we should just remove all the overrides from
> the arch code instead. If there is a reason, then it should be
> documented.

IIRC, when I added the NMI bits to implement 'in_nmi()' I tried to make
this work across archs. But I found that bits in the preempt count have
been hard coded in assembly all over the place, and to clean it up
looked to be a nightmare.

It probably should be done, but I didn't have the time to do it then.
Perhaps we can revisit it.

In fact, looking at all the archs, I believe all archs override the
PREEMPT_ACTIVE bit, and not one of them uses the default (bit 27).

-- Steve


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