~drizzle-trunk/drizzle/development

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

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

Portions of this file contain modifications contributed and copyrighted by
Google, Inc. Those modifications are gratefully acknowledged and are described
briefly in the InnoDB documentation. The contributions by Google are
incorporated with their permission, and subject to the conditions contained in
the file COPYING.Google.

This program is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free Software
Foundation; version 2 of the License.

This program is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with
this program; if not, write to the Free Software Foundation, Inc., 59 Temple
Place, Suite 330, Boston, MA 02111-1307 USA

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

/******************************************************
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. */
UNIV_INLINE
byte
mutex_test_and_set(
/*===============*/
				/* out: the previous value of lock_word: 0 or
				1 */
	mutex_t*	mutex)	/* in: mutex */
{
#if defined(_WIN32) && defined(UNIV_CAN_USE_X86_ASSEMBLER)
	byte	res;
	byte*	lw;		/* assembler code is used to ensure that
				lock_word is loaded from memory */
	ut_ad(mutex);
	ut_ad(sizeof(byte) == 1);

	lw = &(mutex->lock_word);

	__asm	MOV	ECX, lw
		__asm	MOV	EDX, 1
		__asm	XCHG	DL, BYTE PTR [ECX]
		__asm	MOV	res, DL

		/* The fence below would prevent this thread from
		reading the data structure protected by the mutex
		before the test-and-set operation is committed, but
		the fence is apparently not needed:

		In a posting to comp.arch newsgroup (August 10, 1997)
		Andy Glew said that in P6 a LOCKed instruction like
		XCHG establishes a fence with respect to memory reads
		and writes and thus an explicit fence is not
		needed. In P5 he seemed to agree with a previous
		newsgroup poster that LOCKed instructions serialize
		all instruction execution, and, consequently, also
		memory operations. This is confirmed in Intel Software
		Dev. Manual, Vol. 3. */

		/* mutex_fence(); */

		return(res);
#elif defined(HAVE_GCC_ATOMIC_BUILTINS)
	return __sync_lock_test_and_set(&(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(_WIN32) && defined(UNIV_CAN_USE_X86_ASSEMBLER)
	byte*	lw;		/* assembler code is used to ensure that
				lock_word is loaded from memory */
	ut_ad(mutex);

	lw = &(mutex->lock_word);

	__asm	MOV	EDX, 0
		__asm	MOV	ECX, lw
		__asm	XCHG	DL, BYTE PTR [ECX]
#elif defined(HAVE_GCC_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. */
	__sync_lock_test_and_set(&(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
byte
mutex_get_lock_word(
/*================*/
	const mutex_t*	mutex)	/* in: mutex */
{
	const volatile byte*	ptr;	/* declared volatile to ensure that
					lock_word is loaded from memory */
	ut_ad(mutex);

	ptr = &(mutex->lock_word);

	return(*ptr);
}

/**********************************************************************
Gets the waiters field in a mutex. */
UNIV_INLINE
ulint
mutex_get_waiters(
/*==============*/
				/* out: value to set */
	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. */

#if defined UNIV_DEBUG && !defined UNIV_HOTBACKUP
	mutex->count_using++;
#endif /* UNIV_DEBUG && !UNIV_HOTBACKUP */

	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);
}