/*
* 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 */