/* Copyright (c) 2006, Google Inc.
 * All rights reserved.
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 * 
 *     * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 *     * 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.
 *     * Neither the name of Google Inc. nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS 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 COPYRIGHT
 * OWNER 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.
 *
 * ---
 * Author: Sanjay Ghemawat
 */

#include "config.h"
#include <time.h>       /* For nanosleep() */
#include <sched.h>      /* For sched_yield() */
#ifdef HAVE_UNISTD_H
#include <unistd.h>     /* For nanosleep() on Windows, read() */
#endif
#include <fcntl.h>      /* for open(), O_RDONLY */
#include <string.h>     /* for strncmp */
#include <errno.h>
#include "base/spinlock.h"
//#include "base/sysinfo.h"   /* for NumCPUs() */

// We can do contention-profiling of SpinLocks, but the code is in
// mutex.cc, which is not always linked in with spinlock.  Hence we
// provide this weak definition, which is used if mutex.cc isn't linked in.
ATTRIBUTE_WEAK extern void SubmitSpinLockProfileData(const void *, int64);
void SubmitSpinLockProfileData(const void *, int64) {}

static int adaptive_spin_count = 0;

const base::LinkerInitialized SpinLock::LINKER_INITIALIZED =
    base::LINKER_INITIALIZED;

struct SpinLock_InitHelper {
  SpinLock_InitHelper() {
    // On multi-cpu machines, spin for longer before yielding
    // the processor or sleeping.  Reduces idle time significantly.
    //if (NumCPUs() > 1) {
   //   adaptive_spin_count = 1000;
   // }
  }
};

// Hook into global constructor execution:
// We do not do adaptive spinning before that,
// but nothing lock-intensive should be going on at that time.
static SpinLock_InitHelper init_helper;

void SpinLock::SlowLock() {
  int saved_errno = errno; // save and restore errno for signal safety
  int c = adaptive_spin_count;

  // Spin a few times in the hope that the lock holder releases the lock
  while ((c > 0) && (lockword_ != 0)) {
    c--;
  }

  if (lockword_ == 1) {
    sched_yield();          // Spinning failed. Let's try to be gentle.
  }

  while (Acquire_CompareAndSwap(&lockword_, 0, 1) != 0) {
    // This code was adapted from the ptmalloc2 implementation of
    // spinlocks which would sched_yield() upto 50 times before
    // sleeping once for a few milliseconds.  Mike Burrows suggested
    // just doing one sched_yield() outside the loop and always
    // sleeping after that.  This change helped a great deal on the
    // performance of spinlocks under high contention.  A test program
    // with 10 threads on a dual Xeon (four virtual processors) went
    // from taking 30 seconds to 16 seconds.

    // Sleep for a few milliseconds
    struct timespec tm;
    tm.tv_sec = 0;
    tm.tv_nsec = 2000001;
    nanosleep(&tm, NULL);
  }
  errno = saved_errno;
}