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Date: Tue, 4 Oct 2011 16:15:48 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        linux-rt-users <linux-rt-users@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: Quick review of -rt RCU-related patches
Message-ID: <20111004231548.GH2223@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <20111004174755.GA4762@linux.vnet.ibm.com>
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On Wed, Oct 05, 2011 at 12:12:35AM +0200, Peter Zijlstra wrote:
> 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. 

Ah, thank you for the explanation!

							Thanx, Paul