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_ */