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

Peter Zijlstra
2015-04-16T16:52:56Z

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).