From 7fc26327b75685f37f58d64bdb061460f834f80d Mon Sep 17 00:00:00 2001
From: Peter Zijlstra
Date: Wed, 27 May 2015 11:09:36 +0930
Subject: seqlock: Introduce raw_read_seqcount_latch()

Because with latches there is a strict data dependency on the seq load
we can avoid the rmb in favour of a read_barrier_depends.

Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 include/linux/seqlock.h | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

(limited to 'include')

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 1c0cf3102fdc..890c7ef709d5 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -35,6 +35,7 @@
 #include <linux/spinlock.h>
 #include <linux/preempt.h>
 #include <linux/lockdep.h>
+#include <linux/compiler.h>
 #include <asm/processor.h>
 
 /*
@@ -233,6 +234,11 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
 	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 +290,7 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
  *	unsigned seq, idx;
  *
  *	do {
- *		seq = latch->seq;
- *		smp_rmb();
+ *		seq = lockless_dereference(latch->seq);
  *
  *		idx = seq & 0x01;
  *		entry = data_query(latch->data[idx], ...);
-- 
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