1 /* rwsem-spinlock.c: R/W semaphores: contention handling functions for
2 * generic spinlock implementation
4 * Copyright (c) 2001 David Howells (dhowells@redhat.com).
5 * - Derived partially from idea by Andrea Arcangeli <andrea@suse.de>
6 * - Derived also from comments by Linus
8 #include <linux/rwsem.h>
9 #include <linux/sched.h>
10 #include <linux/module.h>
13 struct list_head list;
14 struct task_struct *task;
16 #define RWSEM_WAITING_FOR_READ 0x00000001
17 #define RWSEM_WAITING_FOR_WRITE 0x00000002
21 * initialise the semaphore
23 void __init_rwsem(struct rw_semaphore *sem, const char *name,
24 struct lock_class_key *key)
26 #ifdef CONFIG_DEBUG_LOCK_ALLOC
28 * Make sure we are not reinitializing a held semaphore:
30 debug_check_no_locks_freed((void *)sem, sizeof(*sem));
31 lockdep_init_map(&sem->dep_map, name, key, 0);
34 spin_lock_init(&sem->wait_lock);
35 INIT_LIST_HEAD(&sem->wait_list);
39 * handle the lock release when processes blocked on it that can now run
40 * - if we come here, then:
41 * - the 'active count' _reached_ zero
42 * - the 'waiting count' is non-zero
43 * - the spinlock must be held by the caller
44 * - woken process blocks are discarded from the list after having task zeroed
45 * - writers are only woken if wakewrite is non-zero
47 static inline struct rw_semaphore *
48 __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
50 struct rwsem_waiter *waiter;
51 struct task_struct *tsk;
54 waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
57 if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
59 goto dont_wake_writers;
62 /* if we are allowed to wake writers try to grant a single write lock
63 * if there's a writer at the front of the queue
64 * - we leave the 'waiting count' incremented to signify potential
67 if (waiter->flags & RWSEM_WAITING_FOR_WRITE) {
69 list_del(&waiter->list);
71 /* Don't touch waiter after ->task has been NULLed */
79 /* grant an infinite number of read locks to the front of the queue */
82 while (waiter->flags & RWSEM_WAITING_FOR_READ) {
83 struct list_head *next = waiter->list.next;
85 list_del(&waiter->list);
92 if (list_empty(&sem->wait_list))
94 waiter = list_entry(next, struct rwsem_waiter, list);
97 sem->activity += woken;
104 * wake a single writer
106 static inline struct rw_semaphore *
107 __rwsem_wake_one_writer(struct rw_semaphore *sem)
109 struct rwsem_waiter *waiter;
110 struct task_struct *tsk;
114 waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
115 list_del(&waiter->list);
120 wake_up_process(tsk);
121 put_task_struct(tsk);
126 * get a read lock on the semaphore
128 void __sched __down_read(struct rw_semaphore *sem)
130 struct rwsem_waiter waiter;
131 struct task_struct *tsk;
133 spin_lock_irq(&sem->wait_lock);
135 if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
138 spin_unlock_irq(&sem->wait_lock);
143 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
145 /* set up my own style of waitqueue */
147 waiter.flags = RWSEM_WAITING_FOR_READ;
148 get_task_struct(tsk);
150 list_add_tail(&waiter.list, &sem->wait_list);
152 /* we don't need to touch the semaphore struct anymore */
153 spin_unlock_irq(&sem->wait_lock);
155 /* wait to be given the lock */
160 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
163 tsk->state = TASK_RUNNING;
169 * trylock for reading -- returns 1 if successful, 0 if contention
171 int __down_read_trylock(struct rw_semaphore *sem)
177 spin_lock_irqsave(&sem->wait_lock, flags);
179 if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
185 spin_unlock_irqrestore(&sem->wait_lock, flags);
191 * get a write lock on the semaphore
192 * - we increment the waiting count anyway to indicate an exclusive lock
194 void __sched __down_write_nested(struct rw_semaphore *sem, int subclass)
196 struct rwsem_waiter waiter;
197 struct task_struct *tsk;
199 spin_lock_irq(&sem->wait_lock);
201 if (sem->activity == 0 && list_empty(&sem->wait_list)) {
204 spin_unlock_irq(&sem->wait_lock);
209 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
211 /* set up my own style of waitqueue */
213 waiter.flags = RWSEM_WAITING_FOR_WRITE;
214 get_task_struct(tsk);
216 list_add_tail(&waiter.list, &sem->wait_list);
218 /* we don't need to touch the semaphore struct anymore */
219 spin_unlock_irq(&sem->wait_lock);
221 /* wait to be given the lock */
226 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
229 tsk->state = TASK_RUNNING;
234 void __sched __down_write(struct rw_semaphore *sem)
236 __down_write_nested(sem, 0);
240 * trylock for writing -- returns 1 if successful, 0 if contention
242 int __down_write_trylock(struct rw_semaphore *sem)
247 spin_lock_irqsave(&sem->wait_lock, flags);
249 if (sem->activity == 0 && list_empty(&sem->wait_list)) {
255 spin_unlock_irqrestore(&sem->wait_lock, flags);
261 * release a read lock on the semaphore
263 void __up_read(struct rw_semaphore *sem)
267 spin_lock_irqsave(&sem->wait_lock, flags);
269 if (--sem->activity == 0 && !list_empty(&sem->wait_list))
270 sem = __rwsem_wake_one_writer(sem);
272 spin_unlock_irqrestore(&sem->wait_lock, flags);
276 * release a write lock on the semaphore
278 void __up_write(struct rw_semaphore *sem)
282 spin_lock_irqsave(&sem->wait_lock, flags);
285 if (!list_empty(&sem->wait_list))
286 sem = __rwsem_do_wake(sem, 1);
288 spin_unlock_irqrestore(&sem->wait_lock, flags);
292 * downgrade a write lock into a read lock
293 * - just wake up any readers at the front of the queue
295 void __downgrade_write(struct rw_semaphore *sem)
299 spin_lock_irqsave(&sem->wait_lock, flags);
302 if (!list_empty(&sem->wait_list))
303 sem = __rwsem_do_wake(sem, 0);
305 spin_unlock_irqrestore(&sem->wait_lock, flags);
308 EXPORT_SYMBOL(__init_rwsem);
309 EXPORT_SYMBOL(__down_read);
310 EXPORT_SYMBOL(__down_read_trylock);
311 EXPORT_SYMBOL(__down_write_nested);
312 EXPORT_SYMBOL(__down_write);
313 EXPORT_SYMBOL(__down_write_trylock);
314 EXPORT_SYMBOL(__up_read);
315 EXPORT_SYMBOL(__up_write);
316 EXPORT_SYMBOL(__downgrade_write);