linux-kernel - Re: [PATCH v5 05/10] seqlock: Better document raw_write_seqcount

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Date:	Mon, 13 Apr 2015 21:47:25 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	rusty@...tcorp.com.au, mathieu.desnoyers@...icios.com,
	oleg@...hat.com, paulmck@...ux.vnet.ibm.com,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	andi@...stfloor.org, rostedt@...dmis.org, tglx@...utronix.de,
	laijs@...fujitsu.com, linux@...izon.com,
	Andrea Arcangeli <aarcange@...hat.com>,
	David Woodhouse <David.Woodhouse@...el.com>,
	Rik van Riel <riel@...hat.com>,
	Michel Lespinasse <walken@...gle.com>
Subject: Re: [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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