Re: Quick review of -rt RCU-related patches

From: Peter Zijlstra
Date: Tue Oct 04 2011 - 18:13:30 EST

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;

res = __get_cpu_var(local_softirq_runner) == current;
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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