linux-kernel - Re: How do I increment a per-CPU variable without warning?

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Message-ID: <20140415222951.GA742@redhat.com>
Date:	Tue, 15 Apr 2014 18:29:51 -0400
From:	Dave Jones <davej@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	cl@...ux.com, linux-kernel@...r.kernel.org, mingo@...nel.org,
	tj@...nel.org, grygorii.strashko@...com, peterz@...radead.org
Subject: Re: How do I increment a per-CPU variable without warning?

On Tue, Apr 15, 2014 at 03:17:55PM -0700, Paul E. McKenney wrote:
 
 > My current admittedly crude workaround is as follows:
 > 
 > 	static inline bool rcu_should_resched(void)
 > 	{
 > 		int t;
 > 
 > 	#ifdef CONFIG_DEBUG_PREEMPT
 > 		preempt_disable();
 > 	#endif /* #ifdef CONFIG_DEBUG_PREEMPT */
 > 		t = __this_cpu_read(rcu_cond_resched_count) + 1;
 > 		if (t < RCU_COND_RESCHED_LIM) {
 > 			__this_cpu_write(rcu_cond_resched_count, t);
 > 	#ifdef CONFIG_DEBUG_PREEMPT
 > 			preempt_enable();
 > 	#endif /* #ifdef CONFIG_DEBUG_PREEMPT */
 > 			return false;
 > 		}
 > 	#ifdef CONFIG_DEBUG_PREEMPT
 > 		preempt_enable();
 > 	#endif /* #ifdef CONFIG_DEBUG_PREEMPT */
 > 		return true;
 > 	}

Won't using DEBUG_PREEMPT instead of just CONFIG_PREEMPT here make this
silently do the wrong thing if preemption is enabled, but debugging isn't ?

I'm not seeing why you need the ifdefs at all, unless the implied
barrier() is a problem ?

	Dave

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