Re: [PATCH 1/3] sched, timer: Remove usages of ACCESS_ONCE in the scheduler

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Apr 15, 2015 at 09:46:01AM +0200, Ingo Molnar wrote:

 > @@ -2088,7 +2088,7 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)
 >  
 >  static void reset_ptenuma_scan(struct task_struct *p)
 >  {
 > -	ACCESS_ONCE(p->mm->numa_scan_seq)++;
 > +	WRITE_ONCE(p->mm->numa_scan_seq, READ_ONCE(p->mm->numa_scan_seq) + 1);
 
vs

	seq = ACCESS_ONCE(p->mm->numa_scan_seq);
	if (p->numa_scan_seq == seq)
		return;
	p->numa_scan_seq = seq;


> So the original ACCESS_ONCE() barriers were misguided to begin with: I 
> think they tried to handle races with the scheduler balancing softirq 
> and tried to avoid having to use atomics for the sequence counter 
> (which would be overkill), but things like ACCESS_ONCE(x)++ never 
> guaranteed atomicity (or even coherency) of the update.
> 
> But since in reality this is only statistical sampling code, all these 
> compiler barriers can be removed I think. Peter, Mel, Rik, do you 
> agree?

ACCESS_ONCE() is not a compiler barrier

The 'read' side uses ACCESS_ONCE() for two purposes:
 - to load the value once, we don't want the seq number to change under
   us for obvious reasons
 - to avoid load tearing and observe weird seq numbers

The update side uses ACCESS_ONCE() to avoid write tearing, and strictly
speaking it should also worry about read-tearing since its not hard
serialized, although its very unlikely to actually have concurrency
(IIRC).