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/* Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved.
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., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA */
/* Synchronization - readers / writer thread locks */
#include "thr_rwlock.h"
int rw_pr_init(rw_pr_lock_t *rwlock)
{
native_mutex_init(&rwlock->lock, NULL);
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 it is wr-only.
Note that from view point of performance it would be better to
signal on the condition variable after unlocking mutex (as it
reduces number of contex switches).
Unfortunately this would mean that such rwlock can't be safely
used by MDL subsystem, which relies on the fact that it is OK
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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