FreeBSD/Linux Kernel Cross Reference
sys/contrib/ck/include/spinlock/dec.h

     /*
      * Copyright 2010-2015 Samy Al Bahra.
      * 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.
     */
    
    #ifndef CK_SPINLOCK_DEC_H
    #define CK_SPINLOCK_DEC_H
    
    #include <ck_backoff.h>
    #include <ck_cc.h>
    #include <ck_elide.h>
    #include <ck_pr.h>
    #include <ck_stdbool.h>
    
    #ifndef CK_F_SPINLOCK_DEC
    #define CK_F_SPINLOCK_DEC
    /*
     * This is similar to the CACAS lock but makes use of an atomic decrement
     * operation to check if the lock value was decremented to 0 from 1. The
     * idea is that a decrement operation is cheaper than a compare-and-swap.
     */
    struct ck_spinlock_dec {
            unsigned int value;
    };
    typedef struct ck_spinlock_dec ck_spinlock_dec_t;
    
    #define CK_SPINLOCK_DEC_INITIALIZER     {1}
    
    CK_CC_INLINE static void
    ck_spinlock_dec_init(struct ck_spinlock_dec *lock)
    {
    
            lock->value = 1;
            ck_pr_barrier();
            return;
    }
    
    CK_CC_INLINE static bool
    ck_spinlock_dec_trylock(struct ck_spinlock_dec *lock)
    {
            unsigned int value;
    
            value = ck_pr_fas_uint(&lock->value, 0);
            ck_pr_fence_lock();
            return value == 1;
    }
    
    CK_CC_INLINE static bool
    ck_spinlock_dec_locked(struct ck_spinlock_dec *lock)
    {
            bool r;
    
            r = ck_pr_load_uint(&lock->value) != 1;
            ck_pr_fence_acquire();
            return r;
    }
    
    CK_CC_INLINE static void
    ck_spinlock_dec_lock(struct ck_spinlock_dec *lock)
    {
            bool r;
    
            for (;;) {
                    /*
                     * Only one thread is guaranteed to decrement lock to 0.
                     * Overflow must be protected against. No more than
                     * UINT_MAX lock requests can happen while the lock is held.
                     */
                    ck_pr_dec_uint_zero(&lock->value, &r);
                    if (r == true)
                            break;
    
                    /* Load value without generating write cycles. */
                    while (ck_pr_load_uint(&lock->value) != 1)
                            ck_pr_stall();
            }
    
            ck_pr_fence_lock();
           return;
   }
   
   CK_CC_INLINE static void
   ck_spinlock_dec_lock_eb(struct ck_spinlock_dec *lock)
   {
           ck_backoff_t backoff = CK_BACKOFF_INITIALIZER;
           bool r;
   
           for (;;) {
                   ck_pr_dec_uint_zero(&lock->value, &r);
                   if (r == true)
                           break;
   
                   while (ck_pr_load_uint(&lock->value) != 1)
                           ck_backoff_eb(&backoff);
           }
   
           ck_pr_fence_lock();
           return;
   }
   
   CK_CC_INLINE static void
   ck_spinlock_dec_unlock(struct ck_spinlock_dec *lock)
   {
   
           ck_pr_fence_unlock();
   
           /*
            * Unconditionally set lock value to 1 so someone can decrement lock
            * to 0.
            */
           ck_pr_store_uint(&lock->value, 1);
           return;
   }
   
   CK_ELIDE_PROTOTYPE(ck_spinlock_dec, ck_spinlock_dec_t,
       ck_spinlock_dec_locked, ck_spinlock_dec_lock,
       ck_spinlock_dec_locked, ck_spinlock_dec_unlock)
   
   CK_ELIDE_TRYLOCK_PROTOTYPE(ck_spinlock_dec, ck_spinlock_dec_t,
       ck_spinlock_dec_locked, ck_spinlock_dec_trylock)
   
   #endif /* CK_F_SPINLOCK_DEC */
   #endif /* CK_SPINLOCK_DEC_H */

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