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). -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/