From 43dc824d06ba231249c3cc9801e917ed33bcf4aa Mon Sep 17 00:00:00 2001
From: Mateusz Guzik <mjguzik@gmail.com>
Date: Wed, 13 Aug 2014 04:47:31 +0200
Subject: [PATCH 1/2] Implement simple sequence counters with memory barriers.

---
 sys/sys/seq.h | 126 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 126 insertions(+)
 create mode 100644 sys/sys/seq.h

diff --git a/sys/sys/seq.h b/sys/sys/seq.h
new file mode 100644
index 0000000..0971aef
--- /dev/null
+++ b/sys/sys/seq.h
@@ -0,0 +1,126 @@
+/*-
+ * Copyright (c) 2014 The FreeBSD Project
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ * $FreeBSD$
+ */
+
+#ifndef _SYS_SEQ_H_
+#define _SYS_SEQ_H_
+
+#ifdef _KERNEL
+
+/*
+ * Typical usage:
+ *
+ * writers:
+ * 	lock_exclusive(&obj->lock);
+ * 	seq_write_begin(&obj->seq);
+ * 	.....
+ * 	seq_write_end(&obj->seq);
+ * 	unlock_exclusive(&obj->unlock);
+ *
+ * readers:
+ * 	obj_t lobj;
+ * 	seq_t seq;
+ *
+ * 	for (;;) {
+ * 		seq = seq_read(&gobj->seq);
+ * 		lobj = gobj;
+ * 		if (seq_consistent(&gobj->seq, seq))
+ * 			break;
+ * 		cpu_spinwait();
+ * 	}
+ * 	foo(lobj);
+ */		
+
+typedef uint32_t seq_t;
+
+/* A hack to get MPASS macro */
+#include <sys/systm.h>
+#include <sys/lock.h>
+
+#include <machine/cpu.h>
+
+static __inline bool
+seq_in_modify(seq_t seqp)
+{
+
+	return (seqp & 1);
+}
+
+static __inline void
+seq_write_begin(seq_t *seqp)
+{
+
+	MPASS(!seq_in_modify(*seqp));
+	(*seqp)++;
+	wmb();
+}
+
+static __inline void
+seq_write_end(seq_t *seqp)
+{
+
+	wmb();
+	(*seqp)++;
+	MPASS(!seq_in_modify(*seqp));
+}
+
+static __inline seq_t
+seq_read(seq_t *seqp)
+{
+	seq_t ret;
+
+	for (;;) {
+		ret = READ_ONCE(*seqp);
+		if (seq_in_modify(ret)) {
+			cpu_spinwait();
+			continue;
+		}
+		break;
+	}
+
+	rmb();
+
+	return (ret);
+}
+
+static __inline seq_t
+seq_consistent_nomb(seq_t *seqp, seq_t oldseqp)
+{
+
+	MPASS(!seq_in_modify(oldseqp));
+	return (*seqp == oldseqp);
+}
+
+static __inline seq_t
+seq_consistent(seq_t *seqp, seq_t oldseqp)
+{
+
+	rmb();
+	return (seq_consistent_nomb(seqp, oldseqp));
+}
+
+#endif	/* _KERNEL */
+#endif	/* _SYS_SEQ_H_ */
-- 
2.0.2