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 <v...@zeniv.linux.org.uk>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Paul McKenney <paul...@linux.vnet.ibm.com>
Suggested-by: Oleg Nesterov <o...@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <pet...@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
--
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