path: root/storage/xtradb/include/sync0rw.ic

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

Copyright (c) 1995, 2011, Oracle and/or its affiliates. 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

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

/**************************************************//**
@file include/sync0rw.ic
The read-write lock (for threads)

Created 9/11/1995 Heikki Tuuri
*******************************************************/

/******************************************************************//**
Lock an rw-lock in shared mode for the current thread. If the rw-lock is
locked in exclusive mode, or there is an exclusive lock request waiting,
the function spins a preset time (controlled by SYNC_SPIN_ROUNDS),
waiting for the lock before suspending the thread. */
UNIV_INTERN
void
rw_lock_s_lock_spin(
/*================*/
	rw_lock_t*	lock,	/*!< in: pointer to rw-lock */
	ulint		pass,	/*!< in: pass value; != 0, if the lock will
				be passed to another thread to unlock */
	const char*	file_name,/*!< in: file name where lock requested */
	ulint		line);	/*!< in: line where requested */
#ifdef UNIV_SYNC_DEBUG
/******************************************************************//**
Inserts the debug information for an rw-lock. */
UNIV_INTERN
void
rw_lock_add_debug_info(
/*===================*/
	rw_lock_t*	lock,		/*!< in: rw-lock */
	ulint		pass,		/*!< in: pass value */
	ulint		lock_type,	/*!< in: lock type */
	const char*	file_name,	/*!< in: file where requested */
	ulint		line);		/*!< in: line where requested */
/******************************************************************//**
Removes a debug information struct for an rw-lock. */
UNIV_INTERN
void
rw_lock_remove_debug_info(
/*======================*/
	rw_lock_t*	lock,		/*!< in: rw-lock */
	ulint		pass,		/*!< in: pass value */
	ulint		lock_type);	/*!< in: lock type */
#endif /* UNIV_SYNC_DEBUG */

/********************************************************************//**
Check if there are threads waiting for the rw-lock.
@return	1 if waiters, 0 otherwise */
UNIV_INLINE
ulint
rw_lock_get_waiters(
/*================*/
	const rw_lock_t*	lock)	/*!< in: rw-lock */
{
	return(lock->waiters);
}

/********************************************************************//**
Sets lock->waiters to 1. It is not an error if lock->waiters is already
1. On platforms where ATOMIC builtins are used this function enforces a
memory barrier. */
UNIV_INLINE
void
rw_lock_set_waiter_flag(
/*====================*/
	rw_lock_t*	lock)	/*!< in/out: rw-lock */
{
#ifdef INNODB_RW_LOCKS_USE_ATOMICS
	os_compare_and_swap_ulint(&lock->waiters, 0, 1);
#else /* INNODB_RW_LOCKS_USE_ATOMICS */
	lock->waiters = 1;
#endif /* INNODB_RW_LOCKS_USE_ATOMICS */
}

/********************************************************************//**
Resets lock->waiters to 0. It is not an error if lock->waiters is already
0. On platforms where ATOMIC builtins are used this function enforces a
memory barrier. */
UNIV_INLINE
void
rw_lock_reset_waiter_flag(
/*======================*/
	rw_lock_t*	lock)	/*!< in/out: rw-lock */
{
#ifdef INNODB_RW_LOCKS_USE_ATOMICS
	os_compare_and_swap_ulint(&lock->waiters, 1, 0);
#else /* INNODB_RW_LOCKS_USE_ATOMICS */
	lock->waiters = 0;
#endif /* INNODB_RW_LOCKS_USE_ATOMICS */
}

/******************************************************************//**
Returns the write-status of the lock - this function made more sense
with the old rw_lock implementation.
@return	RW_LOCK_NOT_LOCKED, RW_LOCK_EX, RW_LOCK_WAIT_EX */
UNIV_INLINE
ulint
rw_lock_get_writer(
/*===============*/
	const rw_lock_t*	lock)	/*!< in: rw-lock */
{
	lint lock_word = lock->lock_word;
	if (lock_word > 0) {
		/* return NOT_LOCKED in s-lock state, like the writer
		member of the old lock implementation. */
		return(RW_LOCK_NOT_LOCKED);
	} else if (((-lock_word) % X_LOCK_DECR) == 0) {
		return(RW_LOCK_EX);
	} else {
                ut_ad(lock_word > -X_LOCK_DECR);
		return(RW_LOCK_WAIT_EX);
	}
}

/******************************************************************//**
Returns the number of readers.
@return	number of readers */
UNIV_INLINE
ulint
rw_lock_get_reader_count(
/*=====================*/
	const rw_lock_t*	lock)	/*!< in: rw-lock */
{
	lint lock_word = lock->lock_word;
	if (lock_word > 0) {
		/* s-locked, no x-waiters */
		return(X_LOCK_DECR - lock_word);
	} else if (lock_word < 0 && lock_word > -X_LOCK_DECR) {
		/* s-locked, with x-waiters */
		return((ulint)(-lock_word));
	}
	return(0);
}

#ifndef INNODB_RW_LOCKS_USE_ATOMICS
UNIV_INLINE
mutex_t*
rw_lock_get_mutex(
/*==============*/
	rw_lock_t*	lock)
{
	return(&(lock->mutex));
}
#endif

/******************************************************************//**
Returns the value of writer_count for the lock. Does not reserve the lock
mutex, so the caller must be sure it is not changed during the call.
@return	value of writer_count */
UNIV_INLINE
ulint
rw_lock_get_x_lock_count(
/*=====================*/
	const rw_lock_t*	lock)	/*!< in: rw-lock */
{
	lint lock_copy = lock->lock_word;
	/* If there is a reader, lock_word is not divisible by X_LOCK_DECR */
	if (lock_copy > 0 || (-lock_copy) % X_LOCK_DECR != 0) {
		return(0);
	}
	return(((-lock_copy) / X_LOCK_DECR) + 1);
}

/******************************************************************//**
Two different implementations for decrementing the lock_word of a rw_lock:
one for systems supporting atomic operations, one for others. This does
does not support recusive x-locks: they should be handled by the caller and
need not be atomic since they are performed by the current lock holder.
Returns true if the decrement was made, false if not.
@return	TRUE if decr occurs */
UNIV_INLINE
ibool
rw_lock_lock_word_decr(
/*===================*/
	rw_lock_t*	lock,		/*!< in/out: rw-lock */
	ulint		amount)		/*!< in: amount to decrement */
{
#ifdef INNODB_RW_LOCKS_USE_ATOMICS
        lint local_lock_word = lock->lock_word;
	while (local_lock_word > 0) {
		if (os_compare_and_swap_lint(&lock->lock_word,
					     local_lock_word,
					     local_lock_word - amount)) {
			return(TRUE);
		}
		local_lock_word = lock->lock_word;
	}
	return(FALSE);
#else /* INNODB_RW_LOCKS_USE_ATOMICS */
	ibool success = FALSE;
	mutex_enter(&(lock->mutex));
	if (lock->lock_word > 0) {
		lock->lock_word -= amount;
		success = TRUE;
	}
	mutex_exit(&(lock->mutex));
	return(success);
#endif /* INNODB_RW_LOCKS_USE_ATOMICS */
}

/******************************************************************//**
Increments lock_word the specified amount and returns new value.
@return	lock->lock_word after increment */
UNIV_INLINE
lint
rw_lock_lock_word_incr(
/*===================*/
	rw_lock_t*	lock,		/*!< in/out: rw-lock */
	ulint		amount)		/*!< in: amount of increment */
{
#ifdef INNODB_RW_LOCKS_USE_ATOMICS
	return(os_atomic_increment_lint(&lock->lock_word, amount));
#else /* INNODB_RW_LOCKS_USE_ATOMICS */
	lint local_lock_word;

	mutex_enter(&(lock->mutex));

	lock->lock_word += amount;
	local_lock_word = lock->lock_word;

	mutex_exit(&(lock->mutex));

        return(local_lock_word);
#endif /* INNODB_RW_LOCKS_USE_ATOMICS */
}

/******************************************************************//**
This function sets the lock->writer_thread and lock->recursive fields.
For platforms where we are using atomic builtins instead of lock->mutex
it sets the lock->writer_thread field using atomics to ensure memory
ordering. Note that it is assumed that the caller of this function
effectively owns the lock i.e.: nobody else is allowed to modify
lock->writer_thread at this point in time.
The protocol is that lock->writer_thread MUST be updated BEFORE the
lock->recursive flag is set. */
UNIV_INLINE
void
rw_lock_set_writer_id_and_recursion_flag(
/*=====================================*/
	rw_lock_t*	lock,		/*!< in/out: lock to work on */
	ibool		recursive)	/*!< in: TRUE if recursion
					allowed */
{
	os_thread_id_t	curr_thread	= os_thread_get_curr_id();

#ifdef INNODB_RW_LOCKS_USE_ATOMICS
	os_thread_id_t	local_thread;
	ibool		success;

	/* Prevent Valgrind warnings about writer_thread being
	uninitialized.  It does not matter if writer_thread is
	uninitialized, because we are comparing writer_thread against
	itself, and the operation should always succeed. */
	UNIV_MEM_VALID(&lock->writer_thread, sizeof lock->writer_thread);

	local_thread = lock->writer_thread;
	success = os_compare_and_swap_thread_id(
		&lock->writer_thread, local_thread, curr_thread);
	ut_a(success);
	lock->recursive = recursive;

#else /* INNODB_RW_LOCKS_USE_ATOMICS */

	mutex_enter(&lock->mutex);
	lock->writer_thread = curr_thread;
	lock->recursive = recursive;
	mutex_exit(&lock->mutex);

#endif /* INNODB_RW_LOCKS_USE_ATOMICS */
}

/******************************************************************//**
Low-level function which tries to lock an rw-lock in s-mode. Performs no
spinning.
@return	TRUE if success */
UNIV_INLINE
ibool
rw_lock_s_lock_low(
/*===============*/
	rw_lock_t*	lock,	/*!< in: pointer to rw-lock */
	ulint		pass __attribute__((unused)),
				/*!< in: pass value; != 0, if the lock will be
				passed to another thread to unlock */
	const char*	file_name, /*!< in: file name where lock requested */
	ulint		line)	/*!< in: line where requested */
{
	/* TODO: study performance of UNIV_LIKELY branch prediction hints. */
	if (!rw_lock_lock_word_decr(lock, 1)) {
		/* Locking did not succeed */
		return(FALSE);
	}

#ifdef UNIV_SYNC_DEBUG
	rw_lock_add_debug_info(lock, pass, RW_LOCK_SHARED, file_name, line);
#endif
	/* These debugging values are not set safely: they may be incorrect
        or even refer to a line that is invalid for the file name. */
	lock->last_s_file_name = file_name;
	lock->last_s_line = line;

	return(TRUE);	/* locking succeeded */
}

/******************************************************************//**
Low-level function which locks an rw-lock in s-mode when we know that it
is possible and none else is currently accessing the rw-lock structure.
Then we can do the locking without reserving the mutex. */
UNIV_INLINE
void
rw_lock_s_lock_direct(
/*==================*/
	rw_lock_t*	lock,		/*!< in/out: rw-lock */
	const char*	file_name,	/*!< in: file name where requested */
	ulint		line)		/*!< in: line where lock requested */
{
	ut_ad(lock->lock_word == X_LOCK_DECR);

	/* Indicate there is a new reader by decrementing lock_word */
	lock->lock_word--;

	lock->last_s_file_name = file_name;
	lock->last_s_line = line;

#ifdef UNIV_SYNC_DEBUG
	rw_lock_add_debug_info(lock, 0, RW_LOCK_SHARED, file_name, line);
#endif
}

/******************************************************************//**
Low-level function which locks an rw-lock in x-mode when we know that it
is not locked and none else is currently accessing the rw-lock structure.
Then we can do the locking without reserving the mutex. */
UNIV_INLINE
void
rw_lock_x_lock_direct(
/*==================*/
	rw_lock_t*	lock,		/*!< in/out: rw-lock */
	const char*	file_name,	/*!< in: file name where requested */
	ulint		line)		/*!< in: line where lock requested */
{
	ut_ad(rw_lock_validate(lock));
	ut_ad(lock->lock_word == X_LOCK_DECR);

	lock->lock_word -= X_LOCK_DECR;
	lock->writer_thread = os_thread_get_curr_id();
	lock->recursive = TRUE;

	lock->last_x_file_name = file_name;
	lock->last_x_line = line;

#ifdef UNIV_SYNC_DEBUG
	rw_lock_add_debug_info(lock, 0, RW_LOCK_EX, file_name, line);
#endif
}

/******************************************************************//**
NOTE! Use the corresponding macro, not directly this function! Lock an
rw-lock in shared mode for the current thread. If the rw-lock is locked
in exclusive mode, or there is an exclusive lock request waiting, the
function spins a preset time (controlled by SYNC_SPIN_ROUNDS), waiting for
the lock, before suspending the thread. */
UNIV_INLINE
void
rw_lock_s_lock_func(
/*================*/
	rw_lock_t*	lock,	/*!< in: pointer to rw-lock */
	ulint		pass,	/*!< in: pass value; != 0, if the lock will
				be passed to another thread to unlock */
	const char*	file_name,/*!< in: file name where lock requested */
	ulint		line)	/*!< in: line where requested */
{
	/* NOTE: As we do not know the thread ids for threads which have
	s-locked a latch, and s-lockers will be served only after waiting
	x-lock requests have been fulfilled, then if this thread already
	owns an s-lock here, it may end up in a deadlock with another thread
	which requests an x-lock here. Therefore, we will forbid recursive
	s-locking of a latch: the following assert will warn the programmer
	of the possibility of this kind of a deadlock. If we want to implement
	safe recursive s-locking, we should keep in a list the thread ids of
	the threads which have s-locked a latch. This would use some CPU
	time. */

#ifdef UNIV_SYNC_DEBUG
	ut_ad(!rw_lock_own(lock, RW_LOCK_SHARED)); /* see NOTE above */
#endif /* UNIV_SYNC_DEBUG */

	/* TODO: study performance of UNIV_LIKELY branch prediction hints. */
	if (rw_lock_s_lock_low(lock, pass, file_name, line)) {

		return; /* Success */
	} else {
		/* Did not succeed, try spin wait */

		rw_lock_s_lock_spin(lock, pass, file_name, line);

		return;
	}
}

/******************************************************************//**
NOTE! Use the corresponding macro, not directly this function! Lock an
rw-lock in exclusive mode for the current thread if the lock can be
obtained immediately.
@return	TRUE if success */
UNIV_INLINE
ibool
rw_lock_x_lock_func_nowait(
/*=======================*/
	rw_lock_t*	lock,	/*!< in: pointer to rw-lock */
	const char*	file_name,/*!< in: file name where lock requested */
	ulint		line)	/*!< in: line where requested */
{
	os_thread_id_t	curr_thread	= os_thread_get_curr_id();

	ibool success;

#ifdef INNODB_RW_LOCKS_USE_ATOMICS
	success = os_compare_and_swap_lint(&lock->lock_word, X_LOCK_DECR, 0);
#else

	success = FALSE;
	mutex_enter(&(lock->mutex));
	if (lock->lock_word == X_LOCK_DECR) {
		lock->lock_word = 0;
		success = TRUE;
	}
	mutex_exit(&(lock->mutex));

#endif
	if (success) {
		rw_lock_set_writer_id_and_recursion_flag(lock, TRUE);

	} else if (lock->recursive
		   && os_thread_eq(lock->writer_thread, curr_thread)) {
		/* Relock: this lock_word modification is safe since no other
		threads can modify (lock, unlock, or reserve) lock_word while
		there is an exclusive writer and this is the writer thread. */
		lock->lock_word -= X_LOCK_DECR;

		ut_ad(((-lock->lock_word) % X_LOCK_DECR) == 0);

	} else {
		/* Failure */
		return(FALSE);
	}
#ifdef UNIV_SYNC_DEBUG
	rw_lock_add_debug_info(lock, 0, RW_LOCK_EX, file_name, line);
#endif

	lock->last_x_file_name = file_name;
	lock->last_x_line = line;

	ut_ad(rw_lock_validate(lock));

	return(TRUE);
}

/******************************************************************//**
Releases a shared mode lock. */
UNIV_INLINE
void
rw_lock_s_unlock_func(
/*==================*/
#ifdef UNIV_SYNC_DEBUG
	ulint		pass,	/*!< in: pass value; != 0, if the lock may have
				been passed to another thread to unlock */
#endif
	rw_lock_t*	lock)	/*!< in/out: rw-lock */
{
	ut_ad((lock->lock_word % X_LOCK_DECR) != 0);

#ifdef UNIV_SYNC_DEBUG
	rw_lock_remove_debug_info(lock, pass, RW_LOCK_SHARED);
#endif

	/* Increment lock_word to indicate 1 less reader */
	if (rw_lock_lock_word_incr(lock, 1) == 0) {

		/* wait_ex waiter exists. It may not be asleep, but we signal
                anyway. We do not wake other waiters, because they can't
                exist without wait_ex waiter and wait_ex waiter goes first.*/
		os_event_set(lock->wait_ex_event);
		sync_array_object_signalled(sync_primary_wait_array);

	}

	ut_ad(rw_lock_validate(lock));

#ifdef UNIV_SYNC_PERF_STAT
	rw_s_exit_count++;
#endif
}

/******************************************************************//**
Releases a shared mode lock when we know there are no waiters and none
else will access the lock during the time this function is executed. */
UNIV_INLINE
void
rw_lock_s_unlock_direct(
/*====================*/
	rw_lock_t*	lock)	/*!< in/out: rw-lock */
{
	ut_ad(lock->lock_word < X_LOCK_DECR);

#ifdef UNIV_SYNC_DEBUG
	rw_lock_remove_debug_info(lock, 0, RW_LOCK_SHARED);
#endif

	/* Decrease reader count by incrementing lock_word */
	lock->lock_word++;

	ut_ad(!lock->waiters);
	ut_ad(rw_lock_validate(lock));
#ifdef UNIV_SYNC_PERF_STAT
	rw_s_exit_count++;
#endif
}

/******************************************************************//**
Releases an exclusive mode lock. */
UNIV_INLINE
void
rw_lock_x_unlock_func(
/*==================*/
#ifdef UNIV_SYNC_DEBUG
	ulint		pass,	/*!< in: pass value; != 0, if the lock may have
				been passed to another thread to unlock */
#endif
	rw_lock_t*	lock)	/*!< in/out: rw-lock */
{
	ut_ad((lock->lock_word % X_LOCK_DECR) == 0);

	/* lock->recursive flag also indicates if lock->writer_thread is
	valid or stale. If we are the last of the recursive callers
	then we must unset lock->recursive flag to indicate that the
	lock->writer_thread is now stale.
	Note that since we still hold the x-lock we can safely read the
	lock_word. */
	if (lock->lock_word == 0) {
		/* Last caller in a possible recursive chain. */
		lock->recursive = FALSE;
		UNIV_MEM_INVALID(&lock->writer_thread,
				 sizeof lock->writer_thread);
	}

#ifdef UNIV_SYNC_DEBUG
	rw_lock_remove_debug_info(lock, pass, RW_LOCK_EX);
#endif

	if (rw_lock_lock_word_incr(lock, X_LOCK_DECR) == X_LOCK_DECR) {
		/* Lock is now free. May have to signal read/write waiters.
                We do not need to signal wait_ex waiters, since they cannot
                exist when there is a writer. */
		if (lock->waiters) {
			rw_lock_reset_waiter_flag(lock);
			os_event_set(lock->event);
			sync_array_object_signalled(sync_primary_wait_array);
		}
	}

	ut_ad(rw_lock_validate(lock));

#ifdef UNIV_SYNC_PERF_STAT
	rw_x_exit_count++;
#endif
}

/******************************************************************//**
Releases an exclusive mode lock when we know there are no waiters, and
none else will access the lock during the time this function is executed. */
UNIV_INLINE
void
rw_lock_x_unlock_direct(
/*====================*/
	rw_lock_t*	lock)	/*!< in/out: rw-lock */
{
	/* Reset the exclusive lock if this thread no longer has an x-mode
	lock */

	ut_ad((lock->lock_word % X_LOCK_DECR) == 0);

	if (lock->lock_word == 0) {
		lock->recursive = FALSE;
		UNIV_MEM_INVALID(&lock->writer_thread,
				 sizeof lock->writer_thread);
	}

#ifdef UNIV_SYNC_DEBUG
	rw_lock_remove_debug_info(lock, 0, RW_LOCK_EX);
#endif

	lock->lock_word += X_LOCK_DECR;

	ut_ad(!lock->waiters);
	ut_ad(rw_lock_validate(lock));

#ifdef UNIV_SYNC_PERF_STAT
	rw_x_exit_count++;
#endif
}