From efd21294ab2c2df39e9dd9afa6625f9092d26559 Mon Sep 17 00:00:00 2001
From: John Baldwin <jhb@FreeBSD.org>
Date: Tue, 11 Nov 2003 23:08:26 +0000
Subject: [PATCH] Oh dear, forgot this file in the turnstile commit.  This
 header defines the turnstile API and includes several comments.

Reminded by:	peter
---
 sys/sys/turnstile.h | 88 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 88 insertions(+)
 create mode 100644 sys/sys/turnstile.h

diff --git a/sys/sys/turnstile.h b/sys/sys/turnstile.h
new file mode 100644
index 000000000000..075e82e7f5d8
--- /dev/null
+++ b/sys/sys/turnstile.h
@@ -0,0 +1,88 @@
+/*
+ * Copyright (c) 2002 John Baldwin <jhb@FreeBSD.org>
+ * All rights reserved.
+ *
+ * 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.
+ * 3. Neither the name of the author nor the names of any co-contributors
+ *    may be used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * 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_TURNSTILE_H_
+#define _SYS_TURNSTILE_H_
+
+/*
+ * Turnstile interface.  Non-sleepable locks use a turnstile for the
+ * queue of threads blocked on them when they are contested.
+ *
+ * A thread calls turnstile_lookup() to look up the proper turnstile in
+ * the hash table.  This function returns a pointer to the turnstile and
+ * locks the associated turnstile chain.  A thread calls turnstile_wait()
+ * when the lock is contested to be put on the queue and block.  If a
+ * thread needs to retry a lock operation instead of blocking, it should
+ * call turnstile_release() to unlock the associated turnstile chain lock.
+ *
+ * When a lock is released, either turnstile_signal() or turnstile_wakeup()
+ * is called to mark blocked threads for a pending wakeup.
+ * turnstile_signal() marks the highest priority blocked thread while
+ * turnstile_wakeup() marks all blocked threads.  The turnstile_signal()
+ * function returns true if the turnstile became empty as a result.  After
+ * the higher level code finishes releasing the lock, turnstile_unpend()
+ * must be called to wakeup the pending thread(s).
+ *
+ * When a lock is acquired that already has at least one thread contested
+ * on it, the new owner of the lock must claim ownership of the turnstile
+ * via turnstile_claim().
+ *
+ * Each thread allocates a turnstile at thread creation time via
+ * turnstile_alloc() and frees it at thread destroy time via
+ * turnstile_free().
+ *
+ * A function can query a turnstile to see if it is empty via
+ * turnstile_empty().  The highest priority thread blocked on a turnstile
+ * can be obtained via turnstile_head().
+ */
+
+struct lock_object;
+struct thread;
+struct turnstile;
+
+#ifdef _KERNEL
+
+void	init_turnstiles(void);
+struct turnstile *turnstile_alloc(void);
+void	turnstile_claim(struct turnstile *);
+void	turnstile_free(struct turnstile *);
+struct turnstile *turnstile_lookup(struct lock_object *);
+void	turnstile_release(struct lock_object *);
+int	turnstile_signal(struct turnstile *);
+void	turnstile_unpend(struct turnstile *);
+void	turnstile_wait(struct turnstile *, struct lock_object *,
+	    struct thread *);
+void	turnstile_wakeup(struct turnstile *);
+struct thread *turnstile_head(struct turnstile *);
+int	turnstile_empty(struct turnstile *);
+
+#endif	/* _KERNEL */
+#endif	/* _SYS_TURNSTILE_H_ */