Messages in this thread |  | | Date | Thu, 16 Apr 2015 18:52:24 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 1/3] sched, timer: Remove usages of ACCESS_ONCE in the scheduler |
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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).
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