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

     /*
      * Copyright 2011-2015 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_BRLOCK_H
    #define CK_BRLOCK_H
    
    /*
     * Big reader spinlocks provide cache-local contention-free read
     * lock acquisition in the absence of writers. This comes at the
     * cost of O(n) write lock acquisition. They were first implemented
     * in the Linux kernel by Ingo Molnar and David S. Miller around the
     * year 2000.
     *
     * This implementation is thread-agnostic which comes at the cost
     * of larger reader objects due to necessary linkage overhead. In
     * order to cut down on TLB pressure, it is recommended to allocate
     * these objects on the same page.
     */
    
    #include <ck_pr.h>
    #include <ck_stdbool.h>
    #include <ck_stddef.h>
    
    struct ck_brlock_reader {
            unsigned int n_readers;
            struct ck_brlock_reader *previous;
            struct ck_brlock_reader *next;
    };
    typedef struct ck_brlock_reader ck_brlock_reader_t;
    
    #define CK_BRLOCK_READER_INITIALIZER {0}
    
    struct ck_brlock {
            struct ck_brlock_reader *readers;
            unsigned int writer;
    };
    typedef struct ck_brlock ck_brlock_t;
    
    #define CK_BRLOCK_INITIALIZER {NULL, false}
    
    CK_CC_INLINE static void
    ck_brlock_init(struct ck_brlock *br)
    {
    
            br->readers = NULL;
            br->writer = false;
            ck_pr_barrier();
            return;
    }
    
    CK_CC_INLINE static void
    ck_brlock_write_lock(struct ck_brlock *br)
    {
            struct ck_brlock_reader *cursor;
    
            /*
             * As the frequency of write acquisitions should be low,
             * there is no point to more advanced contention avoidance.
             */
            while (ck_pr_fas_uint(&br->writer, true) == true)
                    ck_pr_stall();
    
            ck_pr_fence_atomic_load();
    
            /* The reader list is protected under the writer br. */
            for (cursor = br->readers; cursor != NULL; cursor = cursor->next) {
                    while (ck_pr_load_uint(&cursor->n_readers) != 0)
                            ck_pr_stall();
            }
    
            ck_pr_fence_lock();
            return;
    }
    
    CK_CC_INLINE static void
    ck_brlock_write_unlock(struct ck_brlock *br)
   {
   
           ck_pr_fence_unlock();
           ck_pr_store_uint(&br->writer, false);
           return;
   }
   
   CK_CC_INLINE static bool
   ck_brlock_write_trylock(struct ck_brlock *br, unsigned int factor)
   {
           struct ck_brlock_reader *cursor;
           unsigned int steps = 0;
   
           while (ck_pr_fas_uint(&br->writer, true) == true) {
                   if (++steps >= factor)
                           return false;
   
                   ck_pr_stall();
           }
   
           /*
            * We do not require a strict fence here as atomic RMW operations
            * are serializing.
            */
           ck_pr_fence_atomic_load();
   
           for (cursor = br->readers; cursor != NULL; cursor = cursor->next) {
                   while (ck_pr_load_uint(&cursor->n_readers) != 0) {
                           if (++steps >= factor) {
                                   ck_brlock_write_unlock(br);
                                   return false;
                           }
   
                           ck_pr_stall();
                   }
           }
   
           ck_pr_fence_lock();
           return true;
   }
   
   CK_CC_INLINE static void
   ck_brlock_read_register(struct ck_brlock *br, struct ck_brlock_reader *reader)
   {
   
           reader->n_readers = 0;
           reader->previous = NULL;
   
           /* Implicit compiler barrier. */
           ck_brlock_write_lock(br);
   
           reader->next = ck_pr_load_ptr(&br->readers);
           if (reader->next != NULL)
                   reader->next->previous = reader;
           ck_pr_store_ptr(&br->readers, reader);
   
           ck_brlock_write_unlock(br);
           return;
   }
   
   CK_CC_INLINE static void
   ck_brlock_read_unregister(struct ck_brlock *br, struct ck_brlock_reader *reader)
   {
   
           ck_brlock_write_lock(br);
   
           if (reader->next != NULL)
                   reader->next->previous = reader->previous;
   
           if (reader->previous != NULL)
                   reader->previous->next = reader->next;
           else
                   br->readers = reader->next;
   
           ck_brlock_write_unlock(br);
           return;
   }
   
   CK_CC_INLINE static void
   ck_brlock_read_lock(struct ck_brlock *br, struct ck_brlock_reader *reader)
   {
   
           if (reader->n_readers >= 1) {
                   ck_pr_store_uint(&reader->n_readers, reader->n_readers + 1);
                   return;
           }
   
           for (;;) {
                   while (ck_pr_load_uint(&br->writer) == true)
                           ck_pr_stall();
   
   #if defined(__x86__) || defined(__x86_64__)
                   ck_pr_fas_uint(&reader->n_readers, 1);
   
                   /*
                    * Serialize reader counter update with respect to load of
                    * writer.
                    */
                   ck_pr_fence_atomic_load();
   #else
                   ck_pr_store_uint(&reader->n_readers, 1);
   
                   /*
                    * Serialize reader counter update with respect to load of
                    * writer.
                    */
                   ck_pr_fence_store_load();
   #endif
   
                   if (ck_pr_load_uint(&br->writer) == false)
                           break;
   
                   ck_pr_store_uint(&reader->n_readers, 0);
           }
   
           ck_pr_fence_lock();
           return;
   }
   
   CK_CC_INLINE static bool
   ck_brlock_read_trylock(struct ck_brlock *br,
                          struct ck_brlock_reader *reader,
                          unsigned int factor)
   {
           unsigned int steps = 0;
   
           if (reader->n_readers >= 1) {
                   ck_pr_store_uint(&reader->n_readers, reader->n_readers + 1);
                   return true;
           }
   
           for (;;) {
                   while (ck_pr_load_uint(&br->writer) == true) {
                           if (++steps >= factor)
                                   return false;
   
                           ck_pr_stall();
                   }
   
   #if defined(__x86__) || defined(__x86_64__)
                   ck_pr_fas_uint(&reader->n_readers, 1);
   
                   /*
                    * Serialize reader counter update with respect to load of
                    * writer.
                    */
                   ck_pr_fence_atomic_load();
   #else
                   ck_pr_store_uint(&reader->n_readers, 1);
   
                   /*
                    * Serialize reader counter update with respect to load of
                    * writer.
                    */
                   ck_pr_fence_store_load();
   #endif
   
                   if (ck_pr_load_uint(&br->writer) == false)
                           break;
   
                   ck_pr_store_uint(&reader->n_readers, 0);
   
                   if (++steps >= factor)
                           return false;
           }
   
           ck_pr_fence_lock();
           return true;
   }
   
   CK_CC_INLINE static void
   ck_brlock_read_unlock(struct ck_brlock_reader *reader)
   {
   
           ck_pr_fence_unlock();
           ck_pr_store_uint(&reader->n_readers, reader->n_readers - 1);
           return;
   }
   
   #endif /* CK_BRLOCK_H */

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