1/*-
2 * SPDX-License-Identifier: BSD-2-Clause
3 *
4 * Copyright (c) 2002 John Baldwin <jhb@FreeBSD.org>
5 *
6 * Redistribution and use in source and binary forms, with or without
7 * modification, are permitted provided that the following conditions
8 * are met:
9 * 1. Redistributions of source code must retain the above copyright
10 * notice, this list of conditions and the following disclaimer.
11 * 2. Redistributions in binary form must reproduce the above copyright
12 * notice, this list of conditions and the following disclaimer in the
13 * documentation and/or other materials provided with the distribution.
14 *
15 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
16 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
17 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
18 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
19 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
20 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
21 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
22 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
23 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
24 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
25 * SUCH DAMAGE.
26 */
27
28#ifndef _SYS_TURNSTILE_H_
29#define _SYS_TURNSTILE_H_
30
31/*
32 * Turnstile interface. Non-sleepable locks use a turnstile for the
33 * queue of threads blocked on them when they are contested. Each
34 * turnstile contains two sub-queues: one for threads waiting for a
35 * shared, or read, lock, and one for threads waiting for an
36 * exclusive, or write, lock.
37 *
38 * A thread calls turnstile_chain_lock() to lock the turnstile chain
39 * associated with a given lock. A thread calls turnstile_wait() when
40 * the lock is contested to be put on the queue and block. If a thread
41 * calls turnstile_trywait() and decides to retry a lock operation instead
42 * of blocking, it should call turnstile_cancel() to unlock the associated
43 * turnstile chain lock.
44 *
45 * When a lock is released, the thread calls turnstile_lookup() to look
46 * up the turnstile associated with the given lock in the hash table. Then
47 * it calls either turnstile_signal() or turnstile_broadcast() to mark
48 * blocked threads for a pending wakeup. turnstile_signal() marks the
49 * highest priority blocked thread while turnstile_broadcast() marks all
50 * blocked threads. The turnstile_signal() function returns true if the
51 * turnstile became empty as a result. After the higher level code finishes
52 * releasing the lock, turnstile_unpend() must be called to wake up the
53 * pending thread(s) and give up ownership of the turnstile.
54 *
55 * Alternatively, if a thread wishes to relinquish ownership of a lock
56 * without waking up any waiters, it may call turnstile_disown().
57 *
58 * When a lock is acquired that already has at least one thread contested
59 * on it, the new owner of the lock must claim ownership of the turnstile
60 * via turnstile_claim().
61 *
62 * Each thread allocates a turnstile at thread creation via turnstile_alloc()
63 * and releases it at thread destruction via turnstile_free(). Note that
64 * a turnstile is not tied to a specific thread and that the turnstile
65 * released at thread destruction may not be the same turnstile that the
66 * thread allocated when it was created.
67 *
68 * The highest priority thread blocked on a specified queue of a
69 * turnstile can be obtained via turnstile_head(). A given queue can
70 * also be queried to see if it is empty via turnstile_empty().
71 */
72
73struct lock_object;
74struct thread;
75struct turnstile;
76
77#ifdef _KERNEL
78
79/* Which queue to block on or which queue to wakeup one or more threads from. */
80#define TS_EXCLUSIVE_QUEUE 0
81#define TS_SHARED_QUEUE 1
82
83void init_turnstiles(void);
84void turnstile_adjust(struct thread *, u_char);
85struct turnstile *turnstile_alloc(void);
86void turnstile_broadcast(struct turnstile *, int);
87void turnstile_cancel(struct turnstile *);
88void turnstile_chain_lock(struct lock_object *);
89void turnstile_chain_unlock(struct lock_object *);
90void turnstile_claim(struct turnstile *);
91void turnstile_disown(struct turnstile *);
92int turnstile_empty(struct turnstile *ts, int queue);
93void turnstile_free(struct turnstile *);
94struct thread *turnstile_head(struct turnstile *, int);
95struct turnstile *turnstile_lookup(struct lock_object *);
96int turnstile_signal(struct turnstile *, int);
97struct turnstile *turnstile_trywait(struct lock_object *);
98void turnstile_unpend(struct turnstile *);
99void turnstile_wait(struct turnstile *, struct thread *, int);
100bool turnstile_lock(struct turnstile *, struct lock_object **,
101 struct thread **);
102void turnstile_unlock(struct turnstile *, struct lock_object *);
103void turnstile_assert(struct turnstile *);
104#endif /* _KERNEL */
105#endif /* _SYS_TURNSTILE_H_ */