linux-kernel - Re: Quick review of -rt RCU-related patches

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Message-ID: <1317766355.32543.38.camel@twins>
Date:	Wed, 05 Oct 2011 00:12:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Quick review of -rt RCU-related patches

On Wed, 2011-10-05 at 00:05 +0200, Thomas Gleixner wrote:
> > peter_zijlstra-frob-rcu.patch
> > 
> >       Looks OK.  Hmmm...  Should this one go to mainline?
> >       Oh, looks equivalent, actually.  So why the change?
> 
> Peter ? 

-       if (in_irq() || in_serving_softirq()) {
+       if (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_OFFSET)) {


For !rt its equivalent yes, for rt otoh its not:

int in_serving_softirq(void)
{
        int res;

        preempt_disable();
        res = __get_cpu_var(local_softirq_runner) == current;
        preempt_enable();
        return res;
}

However invoke_softirq() will still add SOFTIRQ_OFFSET so we need to
look at that to avoid recursion issues.

The changelog describes this. So this change is a direct consequence of
-rt frobbing the softirq stuff and thus isn't needed upstream. 
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