FreeBSD/Linux Kernel Cross Reference
sys/contrib/ck/include/spinlock/anderson.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_ANDERSON_H
    #define CK_SPINLOCK_ANDERSON_H
    
    #include <ck_cc.h>
    #include <ck_limits.h>
    #include <ck_md.h>
    #include <ck_pr.h>
    #include <ck_stdbool.h>
    
    #ifndef CK_F_SPINLOCK_ANDERSON
    #define CK_F_SPINLOCK_ANDERSON
    /*
     * This is an implementation of Anderson's array-based queuing lock.
     */
    struct ck_spinlock_anderson_thread {
            unsigned int locked;
            unsigned int position;
    };
    typedef struct ck_spinlock_anderson_thread ck_spinlock_anderson_thread_t;
    
    struct ck_spinlock_anderson {
            struct ck_spinlock_anderson_thread *slots;
            unsigned int count;
            unsigned int wrap;
            unsigned int mask;
            char pad[CK_MD_CACHELINE - sizeof(unsigned int) * 3 - sizeof(void *)];
            unsigned int next;
    };
    typedef struct ck_spinlock_anderson ck_spinlock_anderson_t;
    
    CK_CC_INLINE static void
    ck_spinlock_anderson_init(struct ck_spinlock_anderson *lock,
        struct ck_spinlock_anderson_thread *slots,
        unsigned int count)
    {
            unsigned int i;
    
            slots[0].locked = false;
            slots[0].position = 0;
            for (i = 1; i < count; i++) {
                    slots[i].locked = true;
                    slots[i].position = i;
            }
    
            lock->slots = slots;
            lock->count = count;
            lock->mask = count - 1;
            lock->next = 0;
    
            /*
             * If the number of threads is not a power of two then compute
             * appropriate wrap-around value in the case of next slot counter
             * overflow.
             */
            if (count & (count - 1))
                    lock->wrap = (UINT_MAX % count) + 1;
            else
                    lock->wrap = 0;
    
            ck_pr_barrier();
            return;
    }
    
    CK_CC_INLINE static bool
    ck_spinlock_anderson_locked(struct ck_spinlock_anderson *lock)
    {
            unsigned int position;
            bool r;
    
            position = ck_pr_load_uint(&lock->next) & lock->mask;
            r = ck_pr_load_uint(&lock->slots[position].locked);
            ck_pr_fence_acquire();
            return r;
   }
   
   CK_CC_INLINE static void
   ck_spinlock_anderson_lock(struct ck_spinlock_anderson *lock,
       struct ck_spinlock_anderson_thread **slot)
   {
           unsigned int position, next;
           unsigned int count = lock->count;
   
           /*
            * If count is not a power of 2, then it is possible for an overflow
            * to reallocate beginning slots to more than one thread. To avoid this
            * use a compare-and-swap.
            */
           if (lock->wrap != 0) {
                   position = ck_pr_load_uint(&lock->next);
   
                   do {
                           if (position == UINT_MAX)
                                   next = lock->wrap;
                           else
                                   next = position + 1;
                   } while (ck_pr_cas_uint_value(&lock->next, position,
                                                 next, &position) == false);
   
                   position %= count;
           } else {
                   position = ck_pr_faa_uint(&lock->next, 1);
                   position &= lock->mask;
           }
   
           /* Serialize with respect to previous thread's store. */
           ck_pr_fence_load();
   
           /*
            * Spin until slot is marked as unlocked. First slot is initialized to
            * false.
            */
           while (ck_pr_load_uint(&lock->slots[position].locked) == true)
                   ck_pr_stall();
   
           /* Prepare slot for potential re-use by another thread. */
           ck_pr_store_uint(&lock->slots[position].locked, true);
           ck_pr_fence_lock();
   
           *slot = lock->slots + position;
           return;
   }
   
   CK_CC_INLINE static void
   ck_spinlock_anderson_unlock(struct ck_spinlock_anderson *lock,
       struct ck_spinlock_anderson_thread *slot)
   {
           unsigned int position;
   
           ck_pr_fence_unlock();
   
           /* Mark next slot as available. */
           if (lock->wrap == 0)
                   position = (slot->position + 1) & lock->mask;
           else
                   position = (slot->position + 1) % lock->count;
   
           ck_pr_store_uint(&lock->slots[position].locked, false);
           return;
   }
   #endif /* CK_F_SPINLOCK_ANDERSON */
   #endif /* CK_SPINLOCK_ANDERSON_H */

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