innobase/sync/sync0rw.cc

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

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Copyright (c) 2008, Google Inc.
Copyright (c) 2017, 2020, MariaDB Corporation.

Portions of this file contain modifications contributed and copyrighted by
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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.

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this program; if not, write to the Free Software Foundation, Inc.,
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*****************************************************************************/

/**************************************************//**
@file sync/sync0rw.cc
The read-write lock (for thread synchronization)

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

#include "sync0rw.h"
#include "my_cpu.h"
#include <my_sys.h>

/*
	IMPLEMENTATION OF THE RW_LOCK
	=============================
The status of a rw_lock is held in lock_word. The initial value of lock_word is
X_LOCK_DECR. lock_word is decremented by 1 for each s-lock and by X_LOCK_DECR
or 1 for each x-lock. This describes the lock state for each value of lock_word:

lock_word == X_LOCK_DECR:	Unlocked.
X_LOCK_HALF_DECR < lock_word < X_LOCK_DECR:
				S locked, no waiting writers.
				(X_LOCK_DECR - lock_word) is the number
				of S locks.
lock_word == X_LOCK_HALF_DECR:	SX locked, no waiting writers.
0 < lock_word < X_LOCK_HALF_DECR:
				SX locked AND S locked, no waiting writers.
				(X_LOCK_HALF_DECR - lock_word) is the number
				of S locks.
lock_word == 0:			X locked, no waiting writers.
-X_LOCK_HALF_DECR < lock_word < 0:
				S locked, with a waiting writer.
				(-lock_word) is the number of S locks.
lock_word == -X_LOCK_HALF_DECR:	X locked and SX locked, no waiting writers.
-X_LOCK_DECR < lock_word < -X_LOCK_HALF_DECR:
				S locked, with a waiting writer
				which has SX lock.
				-(lock_word + X_LOCK_HALF_DECR) is the number
				of S locks.
lock_word == -X_LOCK_DECR:	X locked with recursive X lock (2 X locks).
-(X_LOCK_DECR + X_LOCK_HALF_DECR) < lock_word < -X_LOCK_DECR:
				X locked. The number of the X locks is:
				2 - (lock_word + X_LOCK_DECR)
lock_word == -(X_LOCK_DECR + X_LOCK_HALF_DECR):
				X locked with recursive X lock (2 X locks)
				and SX locked.
lock_word < -(X_LOCK_DECR + X_LOCK_HALF_DECR):
				X locked and SX locked.
				The number of the X locks is:
				2 - (lock_word + X_LOCK_DECR + X_LOCK_HALF_DECR)

 LOCK COMPATIBILITY MATRIX

      | S|SX| X|
    --+--+--+--+
     S| +| +| -|
    --+--+--+--+
    SX| +| -| -|
    --+--+--+--+
     X| -| -| -|
    --+--+--+--+

The lock_word is always read and updated atomically and consistently, so that
it always represents the state of the lock, and the state of the lock changes
with a single atomic operation. This lock_word holds all of the information
that a thread needs in order to determine if it is eligible to gain the lock
or if it must spin or sleep. The one exception to this is that writer_thread
must be verified before recursive write locks: to solve this scenario, we make
writer_thread readable by all threads, but only writeable by the x-lock or
sx-lock holder.

The other members of the lock obey the following rules to remain consistent:

writer_thread:	Is used only in recursive x-locking or sx-locking.
		This field is 0 at lock creation time and is updated
		when x-lock is acquired or when move_ownership is called.
		A thread is only allowed to set the value of this field to
		it's thread_id i.e.: a thread cannot set writer_thread to
		some other thread's id.
waiters:	May be set to 1 anytime, but to avoid unnecessary wake-up
		signals, it should only be set to 1 when there are threads
		waiting on event. Must be 1 when a writer starts waiting to
		ensure the current x-locking thread sends a wake-up signal
		during unlock. May only be reset to 0 immediately before a
		a wake-up signal is sent to event. On most platforms, a
		memory barrier is required after waiters is set, and before
		verifying lock_word is still held, to ensure some unlocker
		really does see the flags new value.
event:		Threads wait on event for read or writer lock when another
		thread has an x-lock or an x-lock reservation (wait_ex). A
		thread may only	wait on event after performing the following
		actions in order:
		   (1) Record the counter value of event (with os_event_reset).
		   (2) Set waiters to 1.
		   (3) Verify lock_word <= 0.
		(1) must come before (2) to ensure signal is not missed.
		(2) must come before (3) to ensure a signal is sent.
		These restrictions force the above ordering.
		Immediately before sending the wake-up signal, we should:
		   (1) Verify lock_word == X_LOCK_DECR (unlocked)
		   (2) Reset waiters to 0.
wait_ex_event:	A thread may only wait on the wait_ex_event after it has
		performed the following actions in order:
		   (1) Decrement lock_word by X_LOCK_DECR.
		   (2) Record counter value of wait_ex_event (os_event_reset,
		       called from sync_array_reserve_cell).
		   (3) Verify that lock_word < 0.
		(1) must come first to ensures no other threads become reader
		or next writer, and notifies unlocker that signal must be sent.
		(2) must come before (3) to ensure the signal is not missed.
		These restrictions force the above ordering.
		Immediately before sending the wake-up signal, we should:
		   Verify lock_word == 0 (waiting thread holds x_lock)
*/

rw_lock_stats_t		rw_lock_stats;

/* The global list of rw-locks */
ilist<rw_lock_t> rw_lock_list;
ib_mutex_t		rw_lock_list_mutex;

#ifdef UNIV_DEBUG
/******************************************************************//**
Creates a debug info struct. */
static
rw_lock_debug_t*
rw_lock_debug_create(void);
/*======================*/
/******************************************************************//**
Frees a debug info struct. */
static
void
rw_lock_debug_free(
/*===============*/
	rw_lock_debug_t* info);

/******************************************************************//**
Creates a debug info struct.
@return own: debug info struct */
static
rw_lock_debug_t*
rw_lock_debug_create(void)
/*======================*/
{
	return((rw_lock_debug_t*) ut_malloc_nokey(sizeof(rw_lock_debug_t)));
}

/******************************************************************//**
Frees a debug info struct. */
static
void
rw_lock_debug_free(
/*===============*/
	rw_lock_debug_t* info)
{
	ut_free(info);
}
#endif /* UNIV_DEBUG */

/******************************************************************//**
Creates, or rather, initializes an rw-lock object in a specified memory
location (which must be appropriately aligned). The rw-lock is initialized
to the non-locked state. Explicit freeing of the rw-lock with rw_lock_free
is necessary only if the memory block containing it is freed. */
void
rw_lock_create_func(
/*================*/
	rw_lock_t*	lock,		/*!< in: pointer to memory */
#ifdef UNIV_DEBUG
	latch_level_t	level,		/*!< in: level */
#endif /* UNIV_DEBUG */
	const char*	cfile_name,	/*!< in: file name where created */
	unsigned	cline)		/*!< in: file line where created */
{
#if defined(UNIV_DEBUG) && !defined(UNIV_PFS_RWLOCK)
	/* It should have been created in pfs_rw_lock_create_func() */
	new(lock) rw_lock_t();
#endif /* UNIV_DEBUG */

	lock->lock_word = X_LOCK_DECR;
	lock->waiters = 0;

	lock->sx_recursive = 0;
	lock->writer_thread= 0;

#ifdef UNIV_DEBUG
	lock->m_rw_lock = true;

	UT_LIST_INIT(lock->debug_list, &rw_lock_debug_t::list);

	lock->m_id = sync_latch_get_id(sync_latch_get_name(level));
	ut_a(lock->m_id != LATCH_ID_NONE);

	lock->level = level;
#endif /* UNIV_DEBUG */

	lock->cfile_name = cfile_name;

	/* This should hold in practice. If it doesn't then we need to
	split the source file anyway. Or create the locks on lines
	less than 8192. cline is unsigned:13. */
	ut_ad(cline <= ((1U << 13) - 1));
	lock->cline = cline & ((1U << 13) - 1);
	lock->count_os_wait = 0;
	lock->last_x_file_name = "not yet reserved";
	lock->last_x_line = 0;
	lock->event = os_event_create(0);
	lock->wait_ex_event = os_event_create(0);

	lock->is_block_lock = 0;

	ut_d(lock->created = true);

	mutex_enter(&rw_lock_list_mutex);
	rw_lock_list.push_front(*lock);
	mutex_exit(&rw_lock_list_mutex);
}

/******************************************************************//**
Calling this function is obligatory only if the memory buffer containing
the rw-lock is freed. Removes an rw-lock object from the global list. The
rw-lock is checked to be in the non-locked state. */
void
rw_lock_free_func(
/*==============*/
	rw_lock_t*	lock)	/*!< in/out: rw-lock */
{
	ut_ad(rw_lock_validate(lock));
	ut_a(lock->lock_word == X_LOCK_DECR);

	ut_d(lock->created = false);

	mutex_enter(&rw_lock_list_mutex);

	os_event_destroy(lock->event);

	os_event_destroy(lock->wait_ex_event);

	rw_lock_list.remove(*lock);

	mutex_exit(&rw_lock_list_mutex);
}

/******************************************************************//**
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 srv_n_spin_wait_rounds), waiting
for the lock, before suspending the thread. */
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 */
	unsigned	line)	/*!< in: line where requested */
{
	ulint		i = 0;	/* spin round count */
	sync_array_t*	sync_arr;
	lint		spin_count = 0;
	int64_t		count_os_wait = 0;

	/* We reuse the thread id to index into the counter, cache
	it here for efficiency. */

	ut_ad(rw_lock_validate(lock));

	rw_lock_stats.rw_s_spin_wait_count.inc();

lock_loop:

	/* Spin waiting for the writer field to become free */
	HMT_low();
	ulint j = i;
	while (i < srv_n_spin_wait_rounds &&
	       lock->lock_word <= 0) {
		ut_delay(srv_spin_wait_delay);
		i++;
	}

	HMT_medium();
	if (i >= srv_n_spin_wait_rounds) {
		os_thread_yield();
	}

	spin_count += lint(i - j);

	/* We try once again to obtain the lock */
	if (rw_lock_s_lock_low(lock, pass, file_name, line)) {

		if (count_os_wait > 0) {
			lock->count_os_wait +=
				static_cast<uint32_t>(count_os_wait);
			rw_lock_stats.rw_s_os_wait_count.add(count_os_wait);
		}

		rw_lock_stats.rw_s_spin_round_count.add(spin_count);

		return; /* Success */
	} else {

		if (i < srv_n_spin_wait_rounds) {
			goto lock_loop;
		}


		++count_os_wait;

		sync_cell_t*	cell;

		sync_arr = sync_array_get_and_reserve_cell(
				lock, RW_LOCK_S, file_name, line, &cell);

		/* Set waiters before checking lock_word to ensure wake-up
		signal is sent. This may lead to some unnecessary signals. */
		lock->waiters.exchange(1, std::memory_order_acquire);

		if (rw_lock_s_lock_low(lock, pass, file_name, line)) {

			sync_array_free_cell(sync_arr, cell);

			if (count_os_wait > 0) {

				lock->count_os_wait +=
					static_cast<uint32_t>(count_os_wait);

				rw_lock_stats.rw_s_os_wait_count.add(
					count_os_wait);
			}

			rw_lock_stats.rw_s_spin_round_count.add(spin_count);

			return; /* Success */
		}

		/* see comments in trx_commit_low() to
		before_trx_state_committed_in_memory explaining
		this care to invoke the following sync check.*/
#ifndef DBUG_OFF
#ifdef UNIV_DEBUG
		if (lock->get_level() != SYNC_DICT_OPERATION) {
			DEBUG_SYNC_C("rw_s_lock_waiting");
		}
#endif
#endif
		sync_array_wait_event(sync_arr, cell);

		i = 0;

		goto lock_loop;
	}
}

/******************************************************************//**
This function is used in the insert buffer to move the ownership of an
x-latch on a buffer frame to the current thread. The x-latch was set by
the buffer read operation and it protected the buffer frame while the
read was done. The ownership is moved because we want that the current
thread is able to acquire a second x-latch which is stored in an mtr.
This, in turn, is needed to pass the debug checks of index page
operations. */
void
rw_lock_x_lock_move_ownership(
/*==========================*/
	rw_lock_t*	lock)	/*!< in: lock which was x-locked in the
				buffer read */
{
	ut_ad(rw_lock_is_locked(lock, RW_LOCK_X));

	lock->writer_thread = os_thread_get_curr_id();
}

/******************************************************************//**
Function for the next writer to call. Waits for readers to exit.
The caller must have already decremented lock_word by X_LOCK_DECR. */
UNIV_INLINE
void
rw_lock_x_lock_wait_func(
/*=====================*/
	rw_lock_t*	lock,	/*!< in: pointer to rw-lock */
#ifdef UNIV_DEBUG
	ulint		pass,	/*!< in: pass value; != 0, if the lock will
				be passed to another thread to unlock */
#endif
	lint		threshold,/*!< in: threshold to wait for */
	const char*	file_name,/*!< in: file name where lock requested */
	unsigned	line)	/*!< in: line where requested */
{
	ulint		i = 0;
	lint		n_spins = 0;
	sync_array_t*	sync_arr;
	int64_t		count_os_wait = 0;

	ut_ad(lock->lock_word <= threshold);

	HMT_low();
	while (lock->lock_word < threshold) {
		ut_delay(srv_spin_wait_delay);

		if (i < srv_n_spin_wait_rounds) {
			i++;
			continue;
		}

		/* If there is still a reader, then go to sleep.*/
		n_spins += i;

		sync_cell_t*	cell;

		sync_arr = sync_array_get_and_reserve_cell(
			lock, RW_LOCK_X_WAIT, file_name, line, &cell);

		i = 0;

		/* Check lock_word to ensure wake-up isn't missed.*/
		if (lock->lock_word < threshold) {
			++count_os_wait;

			/* Add debug info as it is needed to detect possible
			deadlock. We must add info for WAIT_EX thread for
			deadlock detection to work properly. */
			ut_d(rw_lock_add_debug_info(
					lock, pass, RW_LOCK_X_WAIT,
					file_name, line));

			sync_array_wait_event(sync_arr, cell);

			ut_d(rw_lock_remove_debug_info(
					lock, pass, RW_LOCK_X_WAIT));

			/* It is possible to wake when lock_word < 0.
			We must pass the while-loop check to proceed.*/

		} else {
			sync_array_free_cell(sync_arr, cell);
			break;
		}
	}
	HMT_medium();
	rw_lock_stats.rw_x_spin_round_count.add(n_spins);

	if (count_os_wait > 0) {
		lock->count_os_wait += static_cast<uint32_t>(count_os_wait);
		rw_lock_stats.rw_x_os_wait_count.add(count_os_wait);
	}
}

#ifdef UNIV_DEBUG
# define rw_lock_x_lock_wait(L, P, T, F, O)		\
	rw_lock_x_lock_wait_func(L, P, T, F, O)
#else
# define rw_lock_x_lock_wait(L, P, T, F, O)		\
	rw_lock_x_lock_wait_func(L, T, F, O)
#endif /* UNIV_DBEUG */

/******************************************************************//**
Low-level function for acquiring an exclusive lock.
@return FALSE if did not succeed, TRUE if success. */
UNIV_INLINE
ibool
rw_lock_x_lock_low(
/*===============*/
	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 */
	unsigned	line)	/*!< in: line where requested */
{
	if (rw_lock_lock_word_decr(lock, X_LOCK_DECR, X_LOCK_HALF_DECR)) {

		/* As we are going to write our own thread id in that field it
		must be that the current writer_thread value is not active. */
		ut_a(!lock->writer_thread);

		/* Decrement occurred: we are writer or next-writer. */
		if (!pass)
		{
			lock->writer_thread = os_thread_get_curr_id();
		}

		rw_lock_x_lock_wait(lock, pass, 0, file_name, line);

	} else {
		os_thread_id_t	thread_id = os_thread_get_curr_id();

		/* Decrement failed: An X or SX lock is held by either
		this thread or another. Try to relock. */
		if (!pass && os_thread_eq(lock->writer_thread, thread_id)) {
			/* Other s-locks can be allowed. If it is request x
			recursively while holding sx lock, this x lock should
			be along with the latching-order. */

			/* The existing X or SX lock is from this thread */
			if (rw_lock_lock_word_decr(lock, X_LOCK_DECR, 0)) {
				/* There is at least one SX-lock from this
				thread, but no X-lock. */

				/* Wait for any the other S-locks to be
				released. */
				rw_lock_x_lock_wait(
					lock, pass, -X_LOCK_HALF_DECR,
					file_name, line);

			} else {
				int32_t lock_word = lock->lock_word;
				/* At least one X lock by this thread already
				exists. Add another. */
				if (lock_word == 0
				    || lock_word == -X_LOCK_HALF_DECR) {
					lock->lock_word.fetch_sub(X_LOCK_DECR);
				} else {
					ut_ad(lock_word <= -X_LOCK_DECR);
					lock->lock_word.fetch_sub(1);
				}
			}

		} else {
			/* Another thread locked before us */
			return(FALSE);
		}
	}

	ut_d(rw_lock_add_debug_info(lock, pass, RW_LOCK_X, file_name, line));

	lock->last_x_file_name = file_name;
	lock->last_x_line = line & ((1U << 14) - 1);

	return(TRUE);
}

/******************************************************************//**
Low-level function for acquiring an sx lock.
@return FALSE if did not succeed, TRUE if success. */
ibool
rw_lock_sx_lock_low(
/*================*/
	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 */
	unsigned	line)	/*!< in: line where requested */
{
	if (rw_lock_lock_word_decr(lock, X_LOCK_HALF_DECR, X_LOCK_HALF_DECR)) {

		/* As we are going to write our own thread id in that field it
		must be that the current writer_thread value is not active. */
		ut_a(!lock->writer_thread);

		/* Decrement occurred: we are the SX lock owner. */
		if (!pass)
		{
			lock->writer_thread = os_thread_get_curr_id();
		}

		lock->sx_recursive = 1;
	} else {
		os_thread_id_t	thread_id = os_thread_get_curr_id();

		/* Decrement failed: It already has an X or SX lock by this
		thread or another thread. If it is this thread, relock,
		else fail. */
		if (!pass && os_thread_eq(lock->writer_thread, thread_id)) {
			/* This thread owns an X or SX lock */
			if (lock->sx_recursive++ == 0) {
				/* This thread is making first SX-lock request
				and it must be holding at least one X-lock here
				because:

				* There can't be a WAIT_EX thread because we are
				  the thread which has it's thread_id written in
				  the writer_thread field and we are not waiting.

				* Any other X-lock thread cannot exist because
				  it must update recursive flag only after
				  updating the thread_id. Had there been
				  a concurrent X-locking thread which succeeded
				  in decrementing the lock_word it must have
				  written it's thread_id before setting the
				  recursive flag. As we cleared the if()
				  condition above therefore we must be the only
				  thread working on this lock and it is safe to
				  read and write to the lock_word. */

#ifdef UNIV_DEBUG
				auto lock_word =
#endif
				lock->lock_word.fetch_sub(X_LOCK_HALF_DECR,
							std::memory_order_relaxed);

				ut_ad((lock_word == 0)
				      || ((lock_word <= -X_LOCK_DECR)
					  && (lock_word
					      > -(X_LOCK_DECR
						  + X_LOCK_HALF_DECR))));
			}
		} else {
			/* Another thread locked before us */
			return(FALSE);
		}
	}

	ut_d(rw_lock_add_debug_info(lock, pass, RW_LOCK_SX, file_name, line));

	lock->last_x_file_name = file_name;
	lock->last_x_line = line & ((1U << 14) - 1);

	return(TRUE);
}

/******************************************************************//**
NOTE! Use the corresponding macro, not directly this function! Lock an
rw-lock in exclusive mode for the current thread. If the rw-lock is locked
in shared or exclusive mode, or there is an exclusive lock request waiting,
the function spins a preset time (controlled by srv_n_spin_wait_rounds), waiting
for the lock before suspending the thread. If the same thread has an x-lock
on the rw-lock, locking succeed, with the following exception: if pass != 0,
only a single x-lock may be taken on the lock. NOTE: If the same thread has
an s-lock, locking does not succeed! */
void
rw_lock_x_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 */
	unsigned	line)	/*!< in: line where requested */
{
	ulint		i = 0;
	sync_array_t*	sync_arr;
	lint		spin_count = 0;
	int64_t		count_os_wait = 0;

	ut_ad(rw_lock_validate(lock));
	ut_ad(!rw_lock_own(lock, RW_LOCK_S));

	if (rw_lock_x_lock_low(lock, pass, file_name, line)) {
		/* Locking succeeded */
		return;
	}
	rw_lock_stats.rw_x_spin_wait_count.inc();

lock_loop:

	if (rw_lock_x_lock_low(lock, pass, file_name, line)) {

		if (count_os_wait > 0) {
			lock->count_os_wait +=
				static_cast<uint32_t>(count_os_wait);
			rw_lock_stats.rw_x_os_wait_count.add(count_os_wait);
		}

		rw_lock_stats.rw_x_spin_round_count.add(spin_count);

		/* Locking succeeded */
		return;

	} else {

		/* Spin waiting for the lock_word to become free */
		HMT_low();
		ulint j = i;
		while (i < srv_n_spin_wait_rounds
		       && lock->lock_word <= X_LOCK_HALF_DECR) {
			ut_delay(srv_spin_wait_delay);
			i++;
		}

		HMT_medium();
		spin_count += lint(i - j);

		if (i >= srv_n_spin_wait_rounds) {

			os_thread_yield();

		} else {

			goto lock_loop;
		}
	}

	sync_cell_t*	cell;

	sync_arr = sync_array_get_and_reserve_cell(
			lock, RW_LOCK_X, file_name, line, &cell);

	/* Waiters must be set before checking lock_word, to ensure signal
	is sent. This could lead to a few unnecessary wake-up signals. */
	lock->waiters.exchange(1, std::memory_order_acquire);

	if (rw_lock_x_lock_low(lock, pass, file_name, line)) {
		sync_array_free_cell(sync_arr, cell);

		if (count_os_wait > 0) {
			lock->count_os_wait +=
				static_cast<uint32_t>(count_os_wait);
			rw_lock_stats.rw_x_os_wait_count.add(count_os_wait);
		}

		rw_lock_stats.rw_x_spin_round_count.add(spin_count);

		/* Locking succeeded */
		return;
	}

	++count_os_wait;

	sync_array_wait_event(sync_arr, cell);

	i = 0;

	goto lock_loop;
}

/******************************************************************//**
NOTE! Use the corresponding macro, not directly this function! Lock an
rw-lock in SX 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. If the same thread has an x-lock
on the rw-lock, locking succeed, with the following exception: if pass != 0,
only a single sx-lock may be taken on the lock. NOTE: If the same thread has
an s-lock, locking does not succeed! */
void
rw_lock_sx_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 */
	unsigned	line)	/*!< in: line where requested */

{
	ulint		i = 0;
	sync_array_t*	sync_arr;
	lint		spin_count = 0;
	int64_t		count_os_wait = 0;

	ut_ad(rw_lock_validate(lock));
	ut_ad(!rw_lock_own(lock, RW_LOCK_S));

	if (rw_lock_sx_lock_low(lock, pass, file_name, line)) {
		/* Locking succeeded */
		return;
	}

	rw_lock_stats.rw_sx_spin_wait_count.inc();

lock_loop:

	if (rw_lock_sx_lock_low(lock, pass, file_name, line)) {

		if (count_os_wait > 0) {
			lock->count_os_wait +=
				static_cast<uint32_t>(count_os_wait);
			rw_lock_stats.rw_sx_os_wait_count.add(count_os_wait);
		}

		rw_lock_stats.rw_sx_spin_round_count.add(spin_count);

		/* Locking succeeded */
		return;

	} else {

		/* Spin waiting for the lock_word to become free */
		ulint j = i;
		while (i < srv_n_spin_wait_rounds
		       && lock->lock_word <= X_LOCK_HALF_DECR) {
			ut_delay(srv_spin_wait_delay);
			i++;
		}

		spin_count += lint(i - j);

		if (i >= srv_n_spin_wait_rounds) {

			os_thread_yield();

		} else {

			goto lock_loop;
		}
	}

	sync_cell_t*	cell;

	sync_arr = sync_array_get_and_reserve_cell(
			lock, RW_LOCK_SX, file_name, line, &cell);

	/* Waiters must be set before checking lock_word, to ensure signal
	is sent. This could lead to a few unnecessary wake-up signals. */
	lock->waiters.exchange(1, std::memory_order_acquire);

	if (rw_lock_sx_lock_low(lock, pass, file_name, line)) {

		sync_array_free_cell(sync_arr, cell);

		if (count_os_wait > 0) {
			lock->count_os_wait +=
				static_cast<uint32_t>(count_os_wait);
			rw_lock_stats.rw_sx_os_wait_count.add(count_os_wait);
		}

		rw_lock_stats.rw_sx_spin_round_count.add(spin_count);

		/* Locking succeeded */
		return;
	}

	++count_os_wait;

	sync_array_wait_event(sync_arr, cell);

	i = 0;

	goto lock_loop;
}

#ifdef UNIV_DEBUG

/******************************************************************//**
Checks that the rw-lock has been initialized and that there are no
simultaneous shared and exclusive locks.
@return true */
bool
rw_lock_validate(
/*=============*/
	const rw_lock_t*	lock)	/*!< in: rw-lock */
{
	ut_ad(lock);

	ut_ad(lock->created);

	int32_t lock_word = lock->lock_word;

	ut_ad(lock->waiters < 2);
	ut_ad(lock_word > -(2 * X_LOCK_DECR));
	ut_ad(lock_word <= X_LOCK_DECR);

	return(true);
}

/******************************************************************//**
Checks if somebody has locked the rw-lock in the specified mode.
@return true if locked */
bool
rw_lock_is_locked(
/*==============*/
	rw_lock_t*	lock,		/*!< in: rw-lock */
	ulint		lock_type)	/*!< in: lock type: RW_LOCK_S,
					RW_LOCK_X or RW_LOCK_SX */
{
	ut_ad(rw_lock_validate(lock));

	switch (lock_type) {
	case RW_LOCK_S:
		return(rw_lock_get_reader_count(lock) > 0);

	case RW_LOCK_X:
		return(rw_lock_get_writer(lock) == RW_LOCK_X);

	case RW_LOCK_SX:
		return(rw_lock_get_sx_lock_count(lock) > 0);

	default:
		ut_error;
	}
	return(false);	/* avoid compiler warnings */
}

/******************************************************************//**
Inserts the debug information for an rw-lock. */
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 */
	unsigned	line)		/*!< in: line where requested */
{
	ut_ad(file_name != NULL);

	rw_lock_debug_t*	info = rw_lock_debug_create();

	rw_lock_debug_mutex_enter();

	info->pass	= pass;
	info->line	= line;
	info->lock_type = lock_type;
	info->file_name = file_name;
	info->thread_id = os_thread_get_curr_id();

	UT_LIST_ADD_FIRST(lock->debug_list, info);

	rw_lock_debug_mutex_exit();

	if (pass == 0 && lock_type != RW_LOCK_X_WAIT) {
		int32_t lock_word = lock->lock_word;

		/* Recursive x while holding SX
		(lock_type == RW_LOCK_X && lock_word == -X_LOCK_HALF_DECR)
		is treated as not-relock (new lock). */

		if ((lock_type == RW_LOCK_X
		     && lock_word <  -X_LOCK_HALF_DECR)
		    || (lock_type == RW_LOCK_SX
		       && (lock_word < 0 || lock->sx_recursive == 1))) {

			sync_check_lock_validate(lock);
			sync_check_lock_granted(lock);
		} else {
			sync_check_relock(lock);
		}
	}
}

/******************************************************************//**
Removes a debug information struct for an rw-lock. */
void
rw_lock_remove_debug_info(
/*======================*/
	rw_lock_t*	lock,		/*!< in: rw-lock */
	ulint		pass,		/*!< in: pass value */
	ulint		lock_type)	/*!< in: lock type */
{
	rw_lock_debug_t*	info;

	ut_ad(lock);

	if (pass == 0 && lock_type != RW_LOCK_X_WAIT) {
		sync_check_unlock(lock);
	}

	rw_lock_debug_mutex_enter();

	for (info = UT_LIST_GET_FIRST(lock->debug_list);
	     info != 0;
	     info = UT_LIST_GET_NEXT(list, info)) {

		if (pass == info->pass
		    && (pass != 0
			|| os_thread_eq(info->thread_id,
					os_thread_get_curr_id()))
		    && info->lock_type == lock_type) {

			/* Found! */
			UT_LIST_REMOVE(lock->debug_list, info);

			rw_lock_debug_mutex_exit();

			rw_lock_debug_free(info);

			return;
		}
	}

	ut_error;
}

/******************************************************************//**
Checks if the thread has locked the rw-lock in the specified mode, with
the pass value == 0.
@return TRUE if locked */
bool
rw_lock_own(
/*========*/
	const rw_lock_t*lock,		/*!< in: rw-lock */
	ulint		lock_type)	/*!< in: lock type: RW_LOCK_S,
					RW_LOCK_X */
{
	ut_ad(lock);
	ut_ad(rw_lock_validate(lock));

	const os_thread_id_t thread_id = os_thread_get_curr_id();

	if (!os_thread_eq(lock->writer_thread, thread_id)) {
	} else if (lock_type == RW_LOCK_X && rw_lock_get_x_lock_count(lock)) {
		return TRUE;
	} else if (lock_type == RW_LOCK_SX && rw_lock_get_sx_lock_count(lock)) {
		return TRUE;
	}

	rw_lock_debug_mutex_enter();

	for (const rw_lock_debug_t* info = UT_LIST_GET_FIRST(lock->debug_list);
	     info != NULL;
	     info = UT_LIST_GET_NEXT(list, info)) {

		if (os_thread_eq(info->thread_id, thread_id)
		    && info->pass == 0
		    && info->lock_type == lock_type) {

			rw_lock_debug_mutex_exit();
			/* Found! */

			return(true);
		}
	}
	rw_lock_debug_mutex_exit();

	return(false);
}

/** Checks if the thread has locked the rw-lock in the specified mode, with
the pass value == 0.
@param[in]	lock		rw-lock
@param[in]	flags		specify lock types with OR of the
				rw_lock_flag_t values
@return true if locked */
bool rw_lock_own_flagged(const rw_lock_t* lock, rw_lock_flags_t flags)
{
	ut_ad(rw_lock_validate(lock));

	const os_thread_id_t thread_id = os_thread_get_curr_id();

	if (!os_thread_eq(lock->writer_thread, thread_id)) {
	} else if ((flags & RW_LOCK_FLAG_X)
		   && rw_lock_get_x_lock_count(lock)) {
		return true;
	} else if ((flags & RW_LOCK_FLAG_SX)
		   && rw_lock_get_sx_lock_count(lock)) {
		return true;
	}

	rw_lock_debug_mutex_enter();

	for (rw_lock_debug_t* info = UT_LIST_GET_FIRST(lock->debug_list);
	     info != NULL;
	     info = UT_LIST_GET_NEXT(list, info)) {
		if (!os_thread_eq(info->thread_id, thread_id)
		    || info->pass) {
			continue;
		}

		switch (info->lock_type) {
		case RW_LOCK_S:
			if (!(flags & RW_LOCK_FLAG_S)) {
				continue;
			}
			break;

		case RW_LOCK_X:
			if (!(flags & RW_LOCK_FLAG_X)) {
				continue;
			}
			break;

		case RW_LOCK_SX:
			if (!(flags & RW_LOCK_FLAG_SX)) {
				continue;
			}
			break;
		}

		rw_lock_debug_mutex_exit();
		return true;
	}

	rw_lock_debug_mutex_exit();
	return false;
}

/***************************************************************//**
Prints debug info of currently locked rw-locks. */
void
rw_lock_list_print_info(
/*====================*/
	FILE*	file)		/*!< in: file where to print */
{
	ulint		count = 0;

	mutex_enter(&rw_lock_list_mutex);

	fputs("-------------\n"
	      "RW-LATCH INFO\n"
	      "-------------\n", file);

	for (const rw_lock_t& lock : rw_lock_list) {

		count++;

		if (lock.lock_word != X_LOCK_DECR) {

			fprintf(file, "RW-LOCK: %p ", (void*) &lock);

			if (int32_t waiters= lock.waiters) {
				fprintf(file, " (%d waiters)\n", waiters);
			} else {
				putc('\n', file);
			}

			rw_lock_debug_t* info;

			rw_lock_debug_mutex_enter();

			for (info = UT_LIST_GET_FIRST(lock.debug_list);
			     info != NULL;
			     info = UT_LIST_GET_NEXT(list, info)) {

				rw_lock_debug_print(file, info);
			}

			rw_lock_debug_mutex_exit();
		}
	}

	fprintf(file, "Total number of rw-locks " ULINTPF "\n", count);
	mutex_exit(&rw_lock_list_mutex);
}

/*********************************************************************//**
Prints info of a debug struct. */
void
rw_lock_debug_print(
/*================*/
	FILE*			f,	/*!< in: output stream */
	const rw_lock_debug_t*	info)	/*!< in: debug struct */
{
	ulint	rwt = info->lock_type;

	fprintf(f, "Locked: thread " ULINTPF " file %s line %u  ",
		ulint(info->thread_id),
		sync_basename(info->file_name),
		info->line);

	switch (rwt) {
	case RW_LOCK_S:
		fputs("S-LOCK", f);
		break;
	case RW_LOCK_X:
		fputs("X-LOCK", f);
		break;
	case RW_LOCK_SX:
		fputs("SX-LOCK", f);
		break;
	case RW_LOCK_X_WAIT:
		fputs("WAIT X-LOCK", f);
		break;
	default:
		ut_error;
	}

	if (info->pass != 0) {
		fprintf(f, " pass value %lu", (ulong) info->pass);
	}

	fprintf(f, "\n");
}

/** Print the rw-lock information.
@return the string representation */
std::string
rw_lock_t::to_string() const
{
	/* Note: For X locks it can be locked form multiple places because
	the same thread can call X lock recursively. */

	std::ostringstream	msg;
	bool			written = false;

	ut_ad(rw_lock_validate(this));

	msg << "RW-LATCH: "
	    << "thread id " << os_thread_get_curr_id()
	    << " addr: " << this
	    << " Locked from: ";

	rw_lock_debug_mutex_enter();

	for (rw_lock_debug_t* info = UT_LIST_GET_FIRST(debug_list);
	     info != NULL;
	     info = UT_LIST_GET_NEXT(list, info)) {
		if (!os_thread_eq(info->thread_id, os_thread_get_curr_id())) {
			continue;
		}

		if (written) {
			msg << ", ";
		}

		written = true;

		msg << info->file_name << ":" << info->line;
	}

	rw_lock_debug_mutex_exit();

	return(msg.str());
}
#endif /* UNIV_DEBUG */