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Subject: Re: [PATCH] rcutorture: add random preemption
From: Peter Zijlstra <peterz@infradead.org>
To: paulmck@linux.vnet.ibm.com
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>, Ingo Molnar <mingo@elte.hu>,
       LKML <linux-kernel@vger.kernel.org>
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Date: Mon, 21 Jun 2010 18:50:11 +0200
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On Mon, 2010-06-21 at 09:43 -0700, Paul E. McKenney wrote:
> > +#ifdef CONFIG_PREEMPT
> > +     if (!preempt_count() && !(rcu_random(rrsp) % (nrealreaders * 20000)))
> > +             preempt_schedule();
> > +#endif
> 
> This one scared me for a bit -- then I realized that preempt_schedule()
> won't actually schedule if preemption is in any way disabled.  So the
> above really is OK, because Classic RCU and RCU-bh disable preemption.
> 
> So, should we have a comment to this effect, or is my hypersensitivity to
> RCU semantics unique to me? 

Well it seems to do a !preempt_count() test too, so I wouldn't worry too
much about it, still using preempt_schedule() doesn't seem right, why
not use cond_resched()?