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

     /*
      * Copyright 2013 John Wittrock.
      * Copyright 2013-2015 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_PFLOCK_H
    #define CK_PFLOCK_H
    
    /*
     * This is an implementation of phase-fair locks derived from the work
     * described in:
     *    Brandenburg, B. and Anderson, J. 2010. Spin-Based
     *    Reader-Writer Synchronization for Multiprocessor Real-Time Systems
     */
    
    #include <ck_cc.h>
    #include <ck_pr.h>
    
    struct ck_pflock {
            uint32_t rin;
            uint32_t rout;
            uint32_t win;
            uint32_t wout;
    };
    typedef struct ck_pflock ck_pflock_t;
    
    #define CK_PFLOCK_LSB   0xFFFFFFF0
    #define CK_PFLOCK_RINC  0x100           /* Reader increment value. */
    #define CK_PFLOCK_WBITS 0x3             /* Writer bits in reader. */
    #define CK_PFLOCK_PRES  0x2             /* Writer present bit. */
    #define CK_PFLOCK_PHID  0x1             /* Phase ID bit. */
    
    #define CK_PFLOCK_INITIALIZER {0, 0, 0, 0}
    
    CK_CC_INLINE static void
    ck_pflock_init(struct ck_pflock *pf)
    {
    
            pf->rin = 0;
            pf->rout = 0;
            pf->win = 0;
            pf->wout = 0;
            ck_pr_barrier();
    
            return;
    }
    
    CK_CC_INLINE static void
    ck_pflock_write_unlock(ck_pflock_t *pf)
    {
    
            ck_pr_fence_unlock();
    
            /* Migrate from write phase to read phase. */
            ck_pr_and_32(&pf->rin, CK_PFLOCK_LSB);
    
            /* Allow other writers to continue. */
            ck_pr_faa_32(&pf->wout, 1);
            return;
    }
    
    CK_CC_INLINE static void
    ck_pflock_write_lock(ck_pflock_t *pf)
    {
            uint32_t ticket;
    
            /* Acquire ownership of write-phase. */
            ticket = ck_pr_faa_32(&pf->win, 1);
            while (ck_pr_load_32(&pf->wout) != ticket)
                    ck_pr_stall();
    
            /*
             * Acquire ticket on read-side in order to allow them
             * to flush. Indicates to any incoming reader that a
             * write-phase is pending.
             */
            ticket = ck_pr_faa_32(&pf->rin,
               (ticket & CK_PFLOCK_PHID) | CK_PFLOCK_PRES);
   
           /* Wait for any pending readers to flush. */
           while (ck_pr_load_32(&pf->rout) != ticket)
                   ck_pr_stall();
   
           ck_pr_fence_lock();
           return;
   }
   
   CK_CC_INLINE static void
   ck_pflock_read_unlock(ck_pflock_t *pf)
   {
   
           ck_pr_fence_unlock();
           ck_pr_faa_32(&pf->rout, CK_PFLOCK_RINC);
           return;
   }
   
   CK_CC_INLINE static void
   ck_pflock_read_lock(ck_pflock_t *pf)
   {
           uint32_t w;
   
           /*
            * If no writer is present, then the operation has completed
            * successfully.
            */
           w = ck_pr_faa_32(&pf->rin, CK_PFLOCK_RINC) & CK_PFLOCK_WBITS;
           if (w == 0)
                   goto leave;
   
           /* Wait for current write phase to complete. */
           while ((ck_pr_load_32(&pf->rin) & CK_PFLOCK_WBITS) == w)
                   ck_pr_stall();
   
   leave:
           /* Acquire semantics with respect to readers. */
           ck_pr_fence_lock();
           return;
   }
   
   #endif /* CK_PFLOCK_H */

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