text: /*
   * 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 */
  