/*
* Copyright 2010-2012 PathScale, 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:
*
* 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 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 HOLDER 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.
*/
/**
* guard.cc: Functions for thread-safe static initialisation.
*
* Static values in C++ can be initialised lazily their first use. This file
* contains functions that are used to ensure that two threads attempting to
* initialize the same static do not call the constructor twice. This is
* important because constructors can have side effects, so calling the
* constructor twice may be very bad.
*
* Statics that require initialisation are protected by a 64-bit value. Any
* platform that can do 32-bit atomic test and set operations can use this
* value as a low-overhead lock. Because statics (in most sane code) are
* accessed far more times than they are initialised, this lock implementation
* is heavily optimised towards the case where the static has already been
* initialised.
*/
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>
#include <assert.h>
#include "atomic.h"
// Older GCC doesn't define __LITTLE_ENDIAN__
#ifndef __LITTLE_ENDIAN__
// If __BYTE_ORDER__ is defined, use that instead
# ifdef __BYTE_ORDER__
# if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
# define __LITTLE_ENDIAN__
# endif
// x86 and ARM are the most common little-endian CPUs, so let's have a
// special case for them (ARM is already special cased). Assume everything
// else is big endian.
# elif defined(__x86_64) || defined(__i386)
# define __LITTLE_ENDIAN__
# endif
#endif
/*
* The least significant bit of the guard variable indicates that the object
* has been initialised, the most significant bit is used for a spinlock.
*/
#ifdef __arm__
// ARM ABI - 32-bit guards.
typedef uint32_t guard_t;
typedef uint32_t guard_lock_t;
static const uint32_t LOCKED = static_cast<guard_t>(1) << 31;
static const uint32_t INITIALISED = 1;
#define LOCK_PART(guard) (guard)
#define INIT_PART(guard) (guard)
#elif defined(_LP64)
typedef uint64_t guard_t;
typedef uint64_t guard_lock_t;
# if defined(__LITTLE_ENDIAN__)
static const guard_t LOCKED = static_cast<guard_t>(1) << 63;
static const guard_t INITIALISED = 1;
# else
static const guard_t LOCKED = 1;
static const guard_t INITIALISED = static_cast<guard_t>(1) << 56;
# endif
#define LOCK_PART(guard) (guard)
#define INIT_PART(guard) (guard)
#else
typedef uint32_t guard_lock_t;
# if defined(__LITTLE_ENDIAN__)
typedef struct {
uint32_t init_half;
uint32_t lock_half;
} guard_t;
static const uint32_t LOCKED = static_cast<guard_lock_t>(1) << 31;
static const uint32_t INITIALISED = 1;
# else
typedef struct {
uint32_t init_half;
uint32_t lock_half;
} guard_t;
static_assert(sizeof(guard_t) == sizeof(uint64_t), "");
static const uint32_t LOCKED = 1;
static const uint32_t INITIALISED = static_cast<guard_lock_t>(1) << 24;
# endif
#define LOCK_PART(guard) (&(guard)->lock_half)
#define INIT_PART(guard) (&(guard)->init_half)
#endif
static const guard_lock_t INITIAL = 0;
/**
* Acquires a lock on a guard, returning 0 if the object has already been
* initialised, and 1 if it has not. If the object is already constructed then
* this function just needs to read a byte from memory and return.
*/
extern "C" int __cxa_guard_acquire(volatile guard_t *guard_object)
{
guard_lock_t old;
// Not an atomic read, doesn't establish a happens-before relationship, but
// if one is already established and we end up seeing an initialised state
// then it's a fast path, otherwise we'll do something more expensive than
// this test anyway...
if (INITIALISED == *INIT_PART(guard_object))
return 0;
// Spin trying to do the initialisation
for (;;)
{
// Loop trying to move the value of the guard from 0 (not
// locked, not initialised) to the locked-uninitialised
// position.
old = __sync_val_compare_and_swap(LOCK_PART(guard_object),
INITIAL, LOCKED);
if (old == INITIAL) {
// Lock obtained. If lock and init bit are
// in separate words, check for init race.
if (INIT_PART(guard_object) == LOCK_PART(guard_object))
return 1;
if (INITIALISED != *INIT_PART(guard_object))
return 1;
// No need for a memory barrier here,
// see first comment.
*LOCK_PART(guard_object) = INITIAL;
return 0;
}
// If lock and init bit are in the same word, check again
// if we are done.
if (INIT_PART(guard_object) == LOCK_PART(guard_object) &&
old == INITIALISED)
return 0;
assert(old == LOCKED);
// Another thread holds the lock.
// If lock and init bit are in different words, check
// if we are done before yielding and looping.
if (INIT_PART(guard_object) != LOCK_PART(guard_object) &&
INITIALISED == *INIT_PART(guard_object))
return 0;
sched_yield();
}
}
/**
* Releases the lock without marking the object as initialised. This function
* is called if initialising a static causes an exception to be thrown.
*/
extern "C" void __cxa_guard_abort(volatile guard_t *guard_object)
{
__attribute__((unused))
bool reset = __sync_bool_compare_and_swap(LOCK_PART(guard_object),
LOCKED, INITIAL);
assert(reset);
}
/**
* Releases the guard and marks the object as initialised. This function is
* called after successful initialisation of a static.
*/
extern "C" void __cxa_guard_release(volatile guard_t *guard_object)
{
guard_lock_t old;
if (INIT_PART(guard_object) == LOCK_PART(guard_object))
old = LOCKED;
else
old = INITIAL;
__attribute__((unused))
bool reset = __sync_bool_compare_and_swap(INIT_PART(guard_object),
old, INITIALISED);
assert(reset);
if (INIT_PART(guard_object) != LOCK_PART(guard_object))
*LOCK_PART(guard_object) = INITIAL;
}
