src/base/spinlock_linux-inl.h

// -*- Mode: C++; c-basic-offset: 2; indent-tabs-mode: nil -*-
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 *
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 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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 *
 * ---
 * This file is a Linux-specific part of spinlock_internal.cc
 */

#include <errno.h>
#include <limits.h>
#include <sched.h>
#include <sys/syscall.h>
#include <time.h>
#include <unistd.h>

#define FUTEX_WAIT 0
#define FUTEX_WAKE 1
#define FUTEX_PRIVATE_FLAG 128

// Note: Instead of making direct system calls that are inlined, we rely
//       on the syscall() function in glibc to do the right thing.

static bool have_futex;
static int futex_private_flag = FUTEX_PRIVATE_FLAG;

namespace {
static struct InitModule {
  InitModule() {
    int x = 0;
    // futexes are ints, so we can use them only when
    // that's the same size as the lockword_ in SpinLock.
    have_futex = (sizeof(Atomic32) == sizeof(int) &&
                  syscall(__NR_futex, &x, FUTEX_WAKE, 1, NULL, NULL, 0) >= 0);
    if (have_futex && syscall(__NR_futex, &x, FUTEX_WAKE | futex_private_flag,
                              1, NULL, NULL, 0) < 0) {
      futex_private_flag = 0;
    }
  }
} init_module;

}  // anonymous namespace


namespace base {
namespace internal {

void SpinLockDelay(volatile Atomic32 *w, int32 value, int loop) {
  if (loop != 0) {
    int save_errno = errno;
    struct timespec tm;
    tm.tv_sec = 0;
    if (have_futex) {
      tm.tv_nsec = base::internal::SuggestedDelayNS(loop);
    } else {
      tm.tv_nsec = 2000001;   // above 2ms so linux 2.4 doesn't spin
    }
    if (have_futex) {
      tm.tv_nsec *= 16;  // increase the delay; we expect explicit wakeups
      syscall(__NR_futex, reinterpret_cast<int*>(const_cast<Atomic32*>(w)),
              FUTEX_WAIT | futex_private_flag, value,
              reinterpret_cast<struct kernel_timespec*>(&tm), NULL, 0);
    } else {
      nanosleep(&tm, NULL);
    }
    errno = save_errno;
  }
}

void SpinLockWake(volatile Atomic32 *w, bool all) {
  if (have_futex) {
    syscall(__NR_futex, reinterpret_cast<int*>(const_cast<Atomic32*>(w)),
            FUTEX_WAKE | futex_private_flag, all ? INT_MAX : 1, NULL, NULL, 0);
  }
}

} // namespace internal
} // namespace base