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/* Copyright (c) 2000, 2025, Oracle and/or its affiliates.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License, version 2.0,
as published by the Free Software Foundation.
This program is designed to work with certain software (including
but not limited to OpenSSL) that is licensed under separate terms,
as designated in a particular file or component or in included license
documentation. The authors of MySQL hereby grant you an additional
permission to link the program and your derivative works with the
separately licensed software that they have either included with
the program or referenced in the documentation.
Without limiting anything contained in the foregoing, this file,
which is part of C Driver for MySQL (Connector/C), is also subject to the
Universal FOSS Exception, version 1.0, a copy of which can be found at
http://oss.oracle.com/licenses/universal-foss-exception.
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, version 2.0, 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., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA */
/**
@file mysys/thr_rwlock.cc
Synchronization - readers / writer thread locks
*/
#include "thr_rwlock.h"
int rw_pr_init(rw_pr_lock_t *rwlock) {
native_mutex_init(&rwlock->lock, nullptr);
native_cond_init(&rwlock->no_active_readers);
rwlock->active_readers = 0;
rwlock->writers_waiting_readers = 0;
rwlock->active_writer = false;
#ifdef SAFE_MUTEX
rwlock->writer_thread = 0;
#endif
return 0;
}
int rw_pr_destroy(rw_pr_lock_t *rwlock) {
native_cond_destroy(&rwlock->no_active_readers);
native_mutex_destroy(&rwlock->lock);
return 0;
}
int rw_pr_rdlock(rw_pr_lock_t *rwlock) {
native_mutex_lock(&rwlock->lock);
/*
The fact that we were able to acquire 'lock' mutex means
that there are no active writers and we can acquire rd-lock.
Increment active readers counter to prevent requests for
wr-lock from succeeding and unlock mutex.
*/
rwlock->active_readers++;
native_mutex_unlock(&rwlock->lock);
return 0;
}
int rw_pr_wrlock(rw_pr_lock_t *rwlock) {
native_mutex_lock(&rwlock->lock);
if (rwlock->active_readers != 0) {
/* There are active readers. We have to wait until they are gone. */
rwlock->writers_waiting_readers++;
while (rwlock->active_readers != 0)
native_cond_wait(&rwlock->no_active_readers, &rwlock->lock);
rwlock->writers_waiting_readers--;
}
/*
We own 'lock' mutex so there is no active writers.
Also there are no active readers.
This means that we can grant wr-lock.
Not releasing 'lock' mutex until unlock will block
both requests for rd and wr-locks.
Set 'active_writer' flag to simplify unlock.
Thanks to the fact wr-lock/unlock in the absence of
contention from readers is essentially mutex lock/unlock
with a few simple checks make this rwlock implementation
wr-lock optimized.
*/
rwlock->active_writer = true;
#ifdef SAFE_MUTEX
rwlock->writer_thread = my_thread_self();
#endif
return 0;
}
int rw_pr_unlock(rw_pr_lock_t *rwlock) {
if (rwlock->active_writer) {
/* We are unlocking wr-lock. */
#ifdef SAFE_MUTEX
rwlock->writer_thread = 0;
#endif
rwlock->active_writer = false;
if (rwlock->writers_waiting_readers) {
/*
Avoid expensive cond signal in case when there is no contention
or if it is write-only.
Note that from a performance point of view it would be better to
signal on the condition variable after unlocking mutex (as it
reduces the number of context switches).
Unfortunately this would mean that such rwlock can't be safely
used by the MDL subsystem, which relies on the fact that it is okay
to destroy rwlock once it is in unlocked state.
*/
native_cond_signal(&rwlock->no_active_readers);
}
native_mutex_unlock(&rwlock->lock);
} else {
/* We are unlocking rd-lock. */
native_mutex_lock(&rwlock->lock);
rwlock->active_readers--;
if (rwlock->active_readers == 0 && rwlock->writers_waiting_readers) {
/*
If we are last reader and there are waiting
writers wake them up.
*/
native_cond_signal(&rwlock->no_active_readers);
}
native_mutex_unlock(&rwlock->lock);
}
return 0;
}
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