Viewing file:  rwsem-spinlock.c (5.24 KB) -rw-r--r--
Select action/file-type:
 (+) |  (+) |  (+) | Code (+) | Session (+) |  (+) | SDB (+) |  (+) |  (+) |  (+) |  (+) |  (+) |
/* 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 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 being released whilst there are 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 flags zeroised
*/
static inline struct rw_semaphore *__rwsem_do_wake(struct rw_semaphore *sem)
{
struct rwsem_waiter *waiter;
int woken;
rwsemtrace(sem,"Entering __rwsem_do_wake");
waiter = list_entry(sem->wait_list.next,struct rwsem_waiter,list);
/* 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);
waiter->flags = 0;
wake_up_process(waiter->task);
goto out;
}
/* grant an infinite number of read locks to the readers at the front of the queue */
woken = 0;
do {
list_del(&waiter->list);
waiter->flags = 0;
wake_up_process(waiter->task);
woken++;
if (list_empty(&sem->wait_list))
break;
waiter = list_entry(sem->wait_list.next,struct rwsem_waiter,list);
} while (waiter->flags&RWSEM_WAITING_FOR_READ);
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;
sem->activity = -1;
waiter = list_entry(sem->wait_list.next,struct rwsem_waiter,list);
list_del(&waiter->list);
waiter->flags = 0;
wake_up_process(waiter->task);
return sem;
}
/*
* get a read lock on the semaphore
*/
void __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;
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.flags)
break;
schedule();
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
}
tsk->state = TASK_RUNNING;
out:
rwsemtrace(sem,"Leaving __down_read");
}
/*
* get a write lock on the semaphore
* - note that we increment the waiting count anyway to indicate an exclusive lock
*/
void __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;
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.flags)
break;
schedule();
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
}
tsk->state = TASK_RUNNING;
out:
rwsemtrace(sem,"Leaving __down_write");
}
/*
* release a read lock on the semaphore
*/
void __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 __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);
spin_unlock(&sem->wait_lock);
rwsemtrace(sem,"Leaving __up_write");
}
EXPORT_SYMBOL(init_rwsem);
EXPORT_SYMBOL(__down_read);
EXPORT_SYMBOL(__down_write);
EXPORT_SYMBOL(__up_read);
EXPORT_SYMBOL(__up_write);
#if RWSEM_DEBUG
EXPORT_SYMBOL(rwsemtrace);
#endif
|