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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Apr 15, 2014 at 03:17:55PM -0700, Paul E. McKenney wrote:
> Hello, Christoph,
> 
> I have a patch that currently uses __this_cpu_inc_return() to increment a
> per-CPU variable, but without preemption disabled.  Of course, given that
> preemption is enabled, it might well end up picking up one CPU's counter,
> adding one to it, then storing the result into some other CPU's counter.
> But this is OK, the test can be probabilistic.  And when I run this
> against v3.14 and earlier, it works fine.
> 
> But now there is 188a81409ff7 (percpu: add preemption checks to
> __this_cpu ops), which gives me lots of splats.
> 
> 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;
> 	}
> 
> This is arguably better than the original __this_cpu_read() because it
> avoids overflow, but I thought I should check to see if there was some
> better way to do this.

you could use raw_cpu_{read,write}(). But note that without the
unconditional preempt_disable() in there your code can read a different
rcu_cond_resched_count than it writes.

So I think you very much want that preempt_disable().