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SubjectRe: [PATCH 1/3] sched, timer: Remove usages of ACCESS_ONCE in the scheduler
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On Thu, 2015-04-16 at 20:24 +0200, Ingo Molnar wrote:
> Would it make sense to add a few comments to the seq field definition
> site(s), about how it's supposed to be accessed - or to the
> READ_ONCE()/WRITE_ONCE() sites, to keep people from wondering?

How about this:

---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5a44371..63fa87f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1794,6 +1794,11 @@ static void task_numa_placement(struct task_struct *p)
u64 runtime, period;
spinlock_t *group_lock = NULL;

+ /*
+ * The p->mm->numa_scan_seq gets updated without
+ * exclusive access. Use READ_ONCE() here to ensure
+ * that the field is read in a single access.
+ */
seq = READ_ONCE(p->mm->numa_scan_seq);
if (p->numa_scan_seq == seq)
return;
@@ -2107,6 +2112,13 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)

static void reset_ptenuma_scan(struct task_struct *p)
{
+ /*
+ * We only did a read acquisition of the mmap sem, so
+ * p->mm->numa_scan_seq is written to without exclusive access.
+ * That's not much of an issue though, since this is just used
+ * for statistical sampling. Use WRITE_ONCE and READ_ONCE, which
+ * are not expensive, to avoid load/store tearing.
+ */
WRITE_ONCE(p->mm->numa_scan_seq, READ_ONCE(p->mm->numa_scan_seq) + 1);
p->mm->numa_scan_offset = 0;
}



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