mariadb/include/sync0sync.ic

/*****************************************************************************

Copyright (c) 1995, 2009, Innobase Oy. All Rights Reserved.
Copyright (c) 2008, Google Inc.

Portions of this file contain modifications contributed and copyrighted by
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*****************************************************************************/

/**************************************************//**
@file include/sync0sync.ic
Mutex, the basic synchronization primitive

Created 9/5/1995 Heikki Tuuri
*******************************************************/

/******************************************************************//**
Sets the waiters field in a mutex. */
UNIV_INTERN
void
mutex_set_waiters(
/*==============*/
	mutex_t*	mutex,	/*!< in: mutex */
	ulint		n);	/*!< in: value to set */
/******************************************************************//**
Reserves a mutex for the current thread. If the mutex is reserved, the
function spins a preset time (controlled by SYNC_SPIN_ROUNDS) waiting
for the mutex before suspending the thread. */
UNIV_INTERN
void
mutex_spin_wait(
/*============*/
	mutex_t*	mutex,		/*!< in: pointer to mutex */
	const char*	file_name,	/*!< in: file name where mutex
					requested */
	ulint		line);		/*!< in: line where requested */
#ifdef UNIV_SYNC_DEBUG
/******************************************************************//**
Sets the debug information for a reserved mutex. */
UNIV_INTERN
void
mutex_set_debug_info(
/*=================*/
	mutex_t*	mutex,		/*!< in: mutex */
	const char*	file_name,	/*!< in: file where requested */
	ulint		line);		/*!< in: line where requested */
#endif /* UNIV_SYNC_DEBUG */
/******************************************************************//**
Releases the threads waiting in the primary wait array for this mutex. */
UNIV_INTERN
void
mutex_signal_object(
/*================*/
	mutex_t*	mutex);	/*!< in: mutex */

/******************************************************************//**
Performs an atomic test-and-set instruction to the lock_word field of a
mutex.
@return	the previous value of lock_word: 0 or 1 */
UNIV_INLINE
byte
mutex_test_and_set(
/*===============*/
	mutex_t*	mutex)	/*!< in: mutex */
{
#if defined(HAVE_ATOMIC_BUILTINS)
	return(os_atomic_test_and_set_byte(&mutex->lock_word, 1));
#else
	ibool	ret;

	ret = os_fast_mutex_trylock(&(mutex->os_fast_mutex));

	if (ret == 0) {
		/* We check that os_fast_mutex_trylock does not leak
		and allow race conditions */
		ut_a(mutex->lock_word == 0);

		mutex->lock_word = 1;
	}

	return((byte)ret);
#endif
}

/******************************************************************//**
Performs a reset instruction to the lock_word field of a mutex. This
instruction also serializes memory operations to the program order. */
UNIV_INLINE
void
mutex_reset_lock_word(
/*==================*/
	mutex_t*	mutex)	/*!< in: mutex */
{
#if defined(HAVE_ATOMIC_BUILTINS)
	/* In theory __sync_lock_release should be used to release the lock.
	Unfortunately, it does not work properly alone. The workaround is
	that more conservative __sync_lock_test_and_set is used instead. */
	os_atomic_test_and_set_byte(&mutex->lock_word, 0);
#else
	mutex->lock_word = 0;

	os_fast_mutex_unlock(&(mutex->os_fast_mutex));
#endif
}

/******************************************************************//**
Gets the value of the lock word. */
UNIV_INLINE
lock_word_t
mutex_get_lock_word(
/*================*/
	const mutex_t*	mutex)	/*!< in: mutex */
{
	ut_ad(mutex);

	return(mutex->lock_word);
}

/******************************************************************//**
Gets the waiters field in a mutex.
@return	value to set */
UNIV_INLINE
ulint
mutex_get_waiters(
/*==============*/
	const mutex_t*	mutex)	/*!< in: mutex */
{
	const volatile ulint*	ptr;	/*!< declared volatile to ensure that
					the value is read from memory */
	ut_ad(mutex);

	ptr = &(mutex->waiters);

	return(*ptr);		/* Here we assume that the read of a single
				word from memory is atomic */
}

/******************************************************************//**
Unlocks a mutex owned by the current thread. */
UNIV_INLINE
void
mutex_exit(
/*=======*/
	mutex_t*	mutex)	/*!< in: pointer to mutex */
{
	ut_ad(mutex_own(mutex));

	ut_d(mutex->thread_id = (os_thread_id_t) ULINT_UNDEFINED);

#ifdef UNIV_SYNC_DEBUG
	sync_thread_reset_level(mutex);
#endif
	mutex_reset_lock_word(mutex);

	/* A problem: we assume that mutex_reset_lock word
	is a memory barrier, that is when we read the waiters
	field next, the read must be serialized in memory
	after the reset. A speculative processor might
	perform the read first, which could leave a waiting
	thread hanging indefinitely.

	Our current solution call every second
	sync_arr_wake_threads_if_sema_free()
	to wake up possible hanging threads if
	they are missed in mutex_signal_object. */

	if (mutex_get_waiters(mutex) != 0) {

		mutex_signal_object(mutex);
	}

#ifdef UNIV_SYNC_PERF_STAT
	mutex_exit_count++;
#endif
}

/******************************************************************//**
Locks a mutex for the current thread. If the mutex is reserved, the function
spins a preset time (controlled by SYNC_SPIN_ROUNDS), waiting for the mutex
before suspending the thread. */
UNIV_INLINE
void
mutex_enter_func(
/*=============*/
	mutex_t*	mutex,		/*!< in: pointer to mutex */
	const char*	file_name,	/*!< in: file name where locked */
	ulint		line)		/*!< in: line where locked */
{
	ut_ad(mutex_validate(mutex));
	ut_ad(!mutex_own(mutex));

	/* Note that we do not peek at the value of lock_word before trying
	the atomic test_and_set; we could peek, and possibly save time. */

	ut_d(mutex->count_using++);

	if (!mutex_test_and_set(mutex)) {
		ut_d(mutex->thread_id = os_thread_get_curr_id());
#ifdef UNIV_SYNC_DEBUG
		mutex_set_debug_info(mutex, file_name, line);
#endif
		return;	/* Succeeded! */
	}

	mutex_spin_wait(mutex, file_name, line);
}