blob: 96255f47b1f853f20ae96072ed913f1e8248da49 [file] [log] [blame]
/* rwsem-spinlock.c: R/W semaphores: contention handling functions for
* generic spinlock implementation
*
* Copyright (c) 2001 David Howells (dhowells@redhat.com).
* - Derived partially from idea by Andrea Arcangeli <andrea@suse.de>
* - Derived also from comments by Linus
*/
#include <linux/rwsem.h>
#include <linux/sched.h>
#include <linux/module.h>
struct rwsem_waiter {
struct list_head list;
struct task_struct *task;
unsigned int flags;
#define RWSEM_WAITING_FOR_READ 0x00000001
#define RWSEM_WAITING_FOR_WRITE 0x00000002
};
#if RWSEM_DEBUG
void rwsemtrace(struct rw_semaphore *sem, const char *str)
{
if (sem->debug)
printk("[%d] %s({%d,%d})\n",
current->pid, str, sem->activity,
list_empty(&sem->wait_list) ? 0 : 1);
}
#endif
/*
* initialise the semaphore
*/
void fastcall init_rwsem(struct rw_semaphore *sem)
{
sem->activity = 0;
spin_lock_init(&sem->wait_lock);
INIT_LIST_HEAD(&sem->wait_list);
#if RWSEM_DEBUG
sem->debug = 0;
#endif
}
/*
* handle the lock release when processes blocked on it that can now run
* - if we come here, then:
* - the 'active count' _reached_ zero
* - the 'waiting count' is non-zero
* - the spinlock must be held by the caller
* - woken process blocks are discarded from the list after having task zeroed
* - writers are only woken if wakewrite is non-zero
*/
static inline struct rw_semaphore *
__rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
{
struct rwsem_waiter *waiter;
struct task_struct *tsk;
int woken;
rwsemtrace(sem, "Entering __rwsem_do_wake");
waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
if (!wakewrite) {
if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
goto out;
goto dont_wake_writers;
}
/* if we are allowed to wake writers try to grant a single write lock
* if there's a writer at the front of the queue
* - we leave the 'waiting count' incremented to signify potential
* contention
*/
if (waiter->flags & RWSEM_WAITING_FOR_WRITE) {
sem->activity = -1;
list_del(&waiter->list);
tsk = waiter->task;
/* Don't touch waiter after ->task has been NULLed */
mb();
waiter->task = NULL;
wake_up_process(tsk);
put_task_struct(tsk);
goto out;
}
/* grant an infinite number of read locks to the front of the queue */
dont_wake_writers:
woken = 0;
while (waiter->flags & RWSEM_WAITING_FOR_READ) {
struct list_head *next = waiter->list.next;
list_del(&waiter->list);
tsk = waiter->task;
mb();
waiter->task = NULL;
wake_up_process(tsk);
put_task_struct(tsk);
woken++;
if (list_empty(&sem->wait_list))
break;
waiter = list_entry(next, struct rwsem_waiter, list);
}
sem->activity += woken;
out:
rwsemtrace(sem, "Leaving __rwsem_do_wake");
return sem;
}
/*
* wake a single writer
*/
static inline struct rw_semaphore *
__rwsem_wake_one_writer(struct rw_semaphore *sem)
{
struct rwsem_waiter *waiter;
struct task_struct *tsk;
sem->activity = -1;
waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
list_del(&waiter->list);
tsk = waiter->task;
mb();
waiter->task = NULL;
wake_up_process(tsk);
put_task_struct(tsk);
return sem;
}
/*
* get a read lock on the semaphore
*/
void fastcall __sched __down_read(struct rw_semaphore *sem)
{
struct rwsem_waiter waiter;
struct task_struct *tsk;
rwsemtrace(sem, "Entering __down_read");
spin_lock(&sem->wait_lock);
if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
/* granted */
sem->activity++;
spin_unlock(&sem->wait_lock);
goto out;
}
tsk = current;
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
/* set up my own style of waitqueue */
waiter.task = tsk;
waiter.flags = RWSEM_WAITING_FOR_READ;
get_task_struct(tsk);
list_add_tail(&waiter.list, &sem->wait_list);
/* we don't need to touch the semaphore struct anymore */
spin_unlock(&sem->wait_lock);
/* wait to be given the lock */
for (;;) {
if (!waiter.task)
break;
schedule();
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
}
tsk->state = TASK_RUNNING;
out:
rwsemtrace(sem, "Leaving __down_read");
}
/*
* trylock for reading -- returns 1 if successful, 0 if contention
*/
int fastcall __down_read_trylock(struct rw_semaphore *sem)
{
int ret = 0;
rwsemtrace(sem, "Entering __down_read_trylock");
spin_lock(&sem->wait_lock);
if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
/* granted */
sem->activity++;
ret = 1;
}
spin_unlock(&sem->wait_lock);
rwsemtrace(sem, "Leaving __down_read_trylock");
return ret;
}
/*
* get a write lock on the semaphore
* - we increment the waiting count anyway to indicate an exclusive lock
*/
void fastcall __sched __down_write(struct rw_semaphore *sem)
{
struct rwsem_waiter waiter;
struct task_struct *tsk;
rwsemtrace(sem, "Entering __down_write");
spin_lock(&sem->wait_lock);
if (sem->activity == 0 && list_empty(&sem->wait_list)) {
/* granted */
sem->activity = -1;
spin_unlock(&sem->wait_lock);
goto out;
}
tsk = current;
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
/* set up my own style of waitqueue */
waiter.task = tsk;
waiter.flags = RWSEM_WAITING_FOR_WRITE;
get_task_struct(tsk);
list_add_tail(&waiter.list, &sem->wait_list);
/* we don't need to touch the semaphore struct anymore */
spin_unlock(&sem->wait_lock);
/* wait to be given the lock */
for (;;) {
if (!waiter.task)
break;
schedule();
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
}
tsk->state = TASK_RUNNING;
out:
rwsemtrace(sem, "Leaving __down_write");
}
/*
* trylock for writing -- returns 1 if successful, 0 if contention
*/
int fastcall __down_write_trylock(struct rw_semaphore *sem)
{
int ret = 0;
rwsemtrace(sem, "Entering __down_write_trylock");
spin_lock(&sem->wait_lock);
if (sem->activity == 0 && list_empty(&sem->wait_list)) {
/* granted */
sem->activity = -1;
ret = 1;
}
spin_unlock(&sem->wait_lock);
rwsemtrace(sem, "Leaving __down_write_trylock");
return ret;
}
/*
* release a read lock on the semaphore
*/
void fastcall __up_read(struct rw_semaphore *sem)
{
rwsemtrace(sem, "Entering __up_read");
spin_lock(&sem->wait_lock);
if (--sem->activity == 0 && !list_empty(&sem->wait_list))
sem = __rwsem_wake_one_writer(sem);
spin_unlock(&sem->wait_lock);
rwsemtrace(sem, "Leaving __up_read");
}
/*
* release a write lock on the semaphore
*/
void fastcall __up_write(struct rw_semaphore *sem)
{
rwsemtrace(sem, "Entering __up_write");
spin_lock(&sem->wait_lock);
sem->activity = 0;
if (!list_empty(&sem->wait_list))
sem = __rwsem_do_wake(sem, 1);
spin_unlock(&sem->wait_lock);
rwsemtrace(sem, "Leaving __up_write");
}
/*
* downgrade a write lock into a read lock
* - just wake up any readers at the front of the queue
*/
void fastcall __downgrade_write(struct rw_semaphore *sem)
{
rwsemtrace(sem, "Entering __downgrade_write");
spin_lock(&sem->wait_lock);
sem->activity = 1;
if (!list_empty(&sem->wait_list))
sem = __rwsem_do_wake(sem, 0);
spin_unlock(&sem->wait_lock);
rwsemtrace(sem, "Leaving __downgrade_write");
}
EXPORT_SYMBOL(init_rwsem);
EXPORT_SYMBOL(__down_read);
EXPORT_SYMBOL(__down_read_trylock);
EXPORT_SYMBOL(__down_write);
EXPORT_SYMBOL(__down_write_trylock);
EXPORT_SYMBOL(__up_read);
EXPORT_SYMBOL(__up_write);
EXPORT_SYMBOL(__downgrade_write);
#if RWSEM_DEBUG
EXPORT_SYMBOL(rwsemtrace);
#endif