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/* Simple reader/writer locks.
Copyright (C) 1994, 1995 Free Software Foundation, Inc.
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; either version 2, or (at
your option) any later version.
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., 675 Mass Ave, Cambridge, MA 02139, USA. */
struct rwlock
{
struct mutex master;
struct condition wakeup;
int readers;
int writers_waiting;
int readers_waiting;
};
/* Get a reader lock on reader-writer lock LOCK for disknode DN */
extern inline void
rwlock_reader_lock (struct rwlock *lock)
{
mutex_lock (&lock->master);
if (lock->readers == -1 || lock->writers_waiting)
{
lock->readers_waiting++;
do
condition_wait (&lock->wakeup, &lock->master);
while (lock->readers == -1 || lock->writers_waiting);
lock->readers_waiting--;
}
lock->readers++;
mutex_unlock (&lock->master);
}
/* Get a writer lock on reader-writer lock LOCK for disknode DN */
extern inline void
rwlock_writer_lock (struct rwlock *lock)
{
mutex_lock (&lock->master);
if (lock->readers)
{
lock->writers_waiting++;
do
condition_wait (&lock->wakeup, &lock->master);
while (lock->readers);
lock->writers_waiting--;
}
lock->readers = -1;
mutex_unlock (&lock->master);
}
/* Release a reader lock on reader-writer lock LOCK for disknode DN */
extern inline void
rwlock_reader_unlock (struct rwlock *lock)
{
mutex_lock (&lock->master);
assert (lock->readers);
lock->readers--;
if (lock->readers_waiting || lock->writers_waiting)
condition_broadcast (&lock->wakeup);
mutex_unlock (&lock->master);
}
/* Release a writer lock on reader-writer lock LOCK for disknode DN */
extern inline void
rwlock_writer_unlock (struct rwlock *lock)
{
mutex_lock (&lock->master);
assert (lock->readers == -1);
lock->readers = 0;
if (lock->readers_waiting || lock->writers_waiting)
condition_broadcast (&lock->wakeup);
mutex_unlock (&lock->master);
}
/* Initialize reader-writer lock LOCK */
extern inline void
rwlock_init (struct rwlock *lock)
{
mutex_init (&lock->master);
condition_init (&lock->wakeup);
lock->readers = 0;
lock->readers_waiting = 0;
lock->writers_waiting = 0;
}
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