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

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Message-ID: <20140416130355.GW4496@linux.vnet.ibm.com>
Date:	Wed, 16 Apr 2014 06:03:55 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Dave Jones <davej@...hat.com>, cl@...ux.com,
	linux-kernel@...r.kernel.org, mingo@...nel.org, tj@...nel.org,
	grygorii.strashko@...com
Subject: Re: How do I increment a per-CPU variable without warning?

On Wed, Apr 16, 2014 at 07:21:48AM +0200, Peter Zijlstra wrote:
> On Tue, Apr 15, 2014 at 08:54:19PM -0700, Paul E. McKenney wrote:
> > But falling back on the old ways of doing this at least looks a bit
> > nicer:
> > 
> > 	static inline bool rcu_should_resched(void)
> > 	{
> > 		int t;
> > 		int *tp = &per_cpu(rcu_cond_resched_count, raw_smp_processor_id());
> > 
> > 		t = ACCESS_ONCE(*tp) + 1;
> > 		if (t < RCU_COND_RESCHED_LIM) {
> 
> <here>
> 
> > 			ACCESS_ONCE(*tp) = t;
> > 			return false;
> > 		}
> > 		return true;
> > 	}
> > 
> > Other thoughts?
> 
> Still broken, if A starts out on CPU1, gets migrated to CPU0 at <here>,
> then B starts the same on CPU1. It is possible for both CPU0 and CPU1 to
> write a different value into your rcu_cond_resched_count.

That is actually OK.  The values written are guaranteed to be between
zero and RCU_COND_RESCHED_LIM-1.  In theory, yes, rcu_should_resched()
could end up failing due to a horribly unlucky sequence of preemptions,
but the probability is -way- lower than that of hardware failure.

However...

> You really want to disable preemption around there. The proper old way
> would've been get_cpu_var()/put_cpu_var().

If you are OK with unconditional disabling of preemption at this point,
that would avoid worrying about probabilities and would be quite a bit
simpler.

So unconditional preempt_disable()/preempt_enable() it is.

							Thanx, Paul

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