[PATCH 11/18] seqcount: Introduce raw_write_seqcount_barrier()

Peter Zijlstra
2015-06-11T12:55:12Z

[-- Attachment #1: peterz-seqcount-new-barrier.patch --]
[-- Type: text/plain, Size: 1750 bytes --]

Introduce raw_write_seqcount_barrier(), a new construct that can be
used to provide write barrier semantics in seqcount read loops instead
of the usual consistency guarantee.

Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/seqlock.h | 42 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 42 insertions(+)

--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -233,6 +233,48 @@ static inline void raw_write_seqcount_en
s->sequence++;
}

+/**
+ * raw_write_seqcount_barrier - do a seq write barrier
+ * @s: pointer to seqcount_t
+ *
+ * This can be used to provide an ordering guarantee instead of the
+ * usual consistency guarantee. It is one wmb cheaper, because we can
+ * collapse the two back-to-back wmb()s.
+ *
+ * seqcount_t seq;
+ * bool X = true, Y = false;
+ *
+ * void read(void)
+ * {
+ * bool x, y;
+ *
+ * do {
+ * int s = read_seqcount_begin(&seq);
+ *
+ * x = X; y = Y;
+ *
+ * } while (read_seqcount_retry(&seq, s));
+ *
+ * BUG_ON(!x && !y);
+ * }
+ *
+ * void write(void)
+ * {
+ * Y = true;
+ *
+ * write_seqcount_begin(seq);
+ * write_seqcount_end(seq);
+ *
+ * X = false;
+ * }
+ */
+static inline void raw_write_seqcount_barrier(seqcount_t *s)
+{
+ s->sequence++;
+ smp_wmb();
+ s->sequence++;
+}
+
/*
* raw_write_seqcount_latch - redirect readers to even/odd copy
* @s: pointer to seqcount_t