FreeBSD/Linux Kernel Cross Reference
sys/sys/turnstile.h

     /*-
      * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
      *
      * 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.
     *
     * 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: releng/12.0/sys/sys/turnstile.h 334546 2018-06-02 22:37:53Z mjg $
     */
    
    #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.  Each
     * turnstile contains two sub-queues: one for threads waiting for a
     * shared, or read, lock, and one for threads waiting for an
     * exclusive, or write, lock.
     *
     * A thread calls turnstile_chain_lock() to lock the turnstile chain
     * associated with a given lock.  A thread calls turnstile_wait() when
     * the lock is contested to be put on the queue and block.  If a thread
     * calls turnstile_trywait() and decides to retry a lock operation instead
     * of blocking, it should call turnstile_cancel() to unlock the associated
     * turnstile chain lock.
     *
     * When a lock is released, the thread calls turnstile_lookup() to look
     * up the turnstile associated with the given lock in the hash table.  Then
     * it calls either turnstile_signal() or turnstile_broadcast() to mark
     * blocked threads for a pending wakeup.  turnstile_signal() marks the
     * highest priority blocked thread while turnstile_broadcast() 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 wake up the
     * pending thread(s) and give up ownership of the turnstile.
     *
     * Alternatively, if a thread wishes to relinquish ownership of a lock
     * without waking up any waiters, it may call turnstile_disown().
     *
     * 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 via turnstile_alloc()
     * and releases it at thread destruction via turnstile_free().  Note that
     * a turnstile is not tied to a specific thread and that the turnstile
     * released at thread destruction may not be the same turnstile that the
     * thread allocated when it was created.
     *
     * The highest priority thread blocked on a specified queue of a
     * turnstile can be obtained via turnstile_head().  A given queue can
     * also be queried to see if it is empty via turnstile_empty().
     */
    
    struct lock_object;
    struct thread;
    struct turnstile;
    
    #ifdef _KERNEL
    
    /* Which queue to block on or which queue to wakeup one or more threads from. */
    #define TS_EXCLUSIVE_QUEUE      0
    #define TS_SHARED_QUEUE         1
    
    void    init_turnstiles(void);
    void    turnstile_adjust(struct thread *, u_char);
    struct turnstile *turnstile_alloc(void);
    void    turnstile_broadcast(struct turnstile *, int);
    void    turnstile_cancel(struct turnstile *);
    void    turnstile_chain_lock(struct lock_object *);
    void    turnstile_chain_unlock(struct lock_object *);
    void    turnstile_claim(struct turnstile *);
    void    turnstile_disown(struct turnstile *);
    int     turnstile_empty(struct turnstile *ts, int queue);
    void    turnstile_free(struct turnstile *);
    struct thread *turnstile_head(struct turnstile *, int);
    struct turnstile *turnstile_lookup(struct lock_object *);
    int     turnstile_signal(struct turnstile *, int);
   struct turnstile *turnstile_trywait(struct lock_object *);
   void    turnstile_unpend(struct turnstile *);
   void    turnstile_wait(struct turnstile *, struct thread *, int);
   struct thread *turnstile_lock(struct turnstile *, struct lock_object **);
   void    turnstile_unlock(struct turnstile *, struct lock_object *);
   void    turnstile_assert(struct turnstile *);
   #endif  /* _KERNEL */
   #endif  /* _SYS_TURNSTILE_H_ */

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