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SubjectRe: [PATCH v5 05/10] seqlock: Better document raw_write_seqcount_latch()
On Mon, Apr 13, 2015 at 09:17:50PM +0200, Peter Zijlstra wrote:
> On Mon, Apr 13, 2015 at 06:32:02PM +0200, Ingo Molnar wrote:
> >
> > Btw., I realize this is just a sample, but couldn't this be written
> > more optimally as:
> >
> > do {
> > seq = READ_ONCE(latch->seq);
> > smp_read_barrier_depends();
> >
> > idx = seq & 0x01;
> > entry = data_query(latch->data[idx], ...);
> >
> > smp_rmb();
> > } while (seq != latch->seq);
> >

> Should we look at introducing yet another seq primitive?

Like so?

---
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -233,6 +233,11 @@ static inline void raw_write_seqcount_en
s->sequence++;
}

+static inline int raw_read_seqcount_latch(seqcount_t *s)
+{
+ return lockless_dereference(s->sequence);
+}
+
/**
* raw_write_seqcount_latch - redirect readers to even/odd copy
* @s: pointer to seqcount_t
@@ -284,8 +289,7 @@ static inline void raw_write_seqcount_en
* unsigned seq, idx;
*
* do {
- * seq = latch->seq;
- * smp_rmb();
+ * seq = lockless_dereference(latch->seq);
*
* idx = seq & 0x01;
* entry = data_query(latch->data[idx], ...);
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -393,7 +393,7 @@ static __always_inline u64 __ktime_get_f
u64 now;

do {
- seq = raw_read_seqcount(&tkf->seq);
+ seq = raw_read_seqcount_latch(&tkf->seq);
tkr = tkf->base + (seq & 0x01);
now = ktime_to_ns(tkr->base) + timekeeping_get_ns(tkr);
} while (read_seqcount_retry(&tkf->seq, seq));

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