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 fastcall init_rwsem(struct rw_semaphore *sem)
26 spin_lock_init(&sem->wait_lock);
27 INIT_LIST_HEAD(&sem->wait_list);
31 * handle the lock release when processes blocked on it that can now run
32 * - if we come here, then:
33 * - the 'active count' _reached_ zero
34 * - the 'waiting count' is non-zero
35 * - the spinlock must be held by the caller
36 * - woken process blocks are discarded from the list after having task zeroed
37 * - writers are only woken if wakewrite is non-zero
39 static inline struct rw_semaphore *
40 __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
42 struct rwsem_waiter *waiter;
43 struct task_struct *tsk;
46 waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
49 if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
51 goto dont_wake_writers;
54 /* if we are allowed to wake writers try to grant a single write lock
55 * if there's a writer at the front of the queue
56 * - we leave the 'waiting count' incremented to signify potential
59 if (waiter->flags & RWSEM_WAITING_FOR_WRITE) {
61 list_del(&waiter->list);
63 /* Don't touch waiter after ->task has been NULLed */
71 /* grant an infinite number of read locks to the front of the queue */
74 while (waiter->flags & RWSEM_WAITING_FOR_READ) {
75 struct list_head *next = waiter->list.next;
77 list_del(&waiter->list);
84 if (list_empty(&sem->wait_list))
86 waiter = list_entry(next, struct rwsem_waiter, list);
89 sem->activity += woken;
96 * wake a single writer
98 static inline struct rw_semaphore *
99 __rwsem_wake_one_writer(struct rw_semaphore *sem)
101 struct rwsem_waiter *waiter;
102 struct task_struct *tsk;
106 waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
107 list_del(&waiter->list);
112 wake_up_process(tsk);
113 put_task_struct(tsk);
118 * get a read lock on the semaphore
120 void fastcall __sched __down_read(struct rw_semaphore *sem)
122 struct rwsem_waiter waiter;
123 struct task_struct *tsk;
125 spin_lock_irq(&sem->wait_lock);
127 if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
130 spin_unlock_irq(&sem->wait_lock);
135 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
137 /* set up my own style of waitqueue */
139 waiter.flags = RWSEM_WAITING_FOR_READ;
140 get_task_struct(tsk);
142 list_add_tail(&waiter.list, &sem->wait_list);
144 /* we don't need to touch the semaphore struct anymore */
145 spin_unlock_irq(&sem->wait_lock);
147 /* wait to be given the lock */
152 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
155 tsk->state = TASK_RUNNING;
161 * trylock for reading -- returns 1 if successful, 0 if contention
163 int fastcall __down_read_trylock(struct rw_semaphore *sem)
169 spin_lock_irqsave(&sem->wait_lock, flags);
171 if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
177 spin_unlock_irqrestore(&sem->wait_lock, flags);
183 * get a write lock on the semaphore
184 * - we increment the waiting count anyway to indicate an exclusive lock
186 void fastcall __sched __down_write(struct rw_semaphore *sem)
188 struct rwsem_waiter waiter;
189 struct task_struct *tsk;
191 spin_lock_irq(&sem->wait_lock);
193 if (sem->activity == 0 && list_empty(&sem->wait_list)) {
196 spin_unlock_irq(&sem->wait_lock);
201 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
203 /* set up my own style of waitqueue */
205 waiter.flags = RWSEM_WAITING_FOR_WRITE;
206 get_task_struct(tsk);
208 list_add_tail(&waiter.list, &sem->wait_list);
210 /* we don't need to touch the semaphore struct anymore */
211 spin_unlock_irq(&sem->wait_lock);
213 /* wait to be given the lock */
218 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
221 tsk->state = TASK_RUNNING;
227 * trylock for writing -- returns 1 if successful, 0 if contention
229 int fastcall __down_write_trylock(struct rw_semaphore *sem)
234 spin_lock_irqsave(&sem->wait_lock, flags);
236 if (sem->activity == 0 && list_empty(&sem->wait_list)) {
242 spin_unlock_irqrestore(&sem->wait_lock, flags);
248 * release a read lock on the semaphore
250 void fastcall __up_read(struct rw_semaphore *sem)
254 spin_lock_irqsave(&sem->wait_lock, flags);
256 if (--sem->activity == 0 && !list_empty(&sem->wait_list))
257 sem = __rwsem_wake_one_writer(sem);
259 spin_unlock_irqrestore(&sem->wait_lock, flags);
263 * release a write lock on the semaphore
265 void fastcall __up_write(struct rw_semaphore *sem)
269 spin_lock_irqsave(&sem->wait_lock, flags);
272 if (!list_empty(&sem->wait_list))
273 sem = __rwsem_do_wake(sem, 1);
275 spin_unlock_irqrestore(&sem->wait_lock, flags);
279 * downgrade a write lock into a read lock
280 * - just wake up any readers at the front of the queue
282 void fastcall __downgrade_write(struct rw_semaphore *sem)
286 spin_lock_irqsave(&sem->wait_lock, flags);
289 if (!list_empty(&sem->wait_list))
290 sem = __rwsem_do_wake(sem, 0);
292 spin_unlock_irqrestore(&sem->wait_lock, flags);
295 EXPORT_SYMBOL(init_rwsem);
296 EXPORT_SYMBOL(__down_read);
297 EXPORT_SYMBOL(__down_read_trylock);
298 EXPORT_SYMBOL(__down_write);
299 EXPORT_SYMBOL(__down_write_trylock);
300 EXPORT_SYMBOL(__up_read);
301 EXPORT_SYMBOL(__up_write);
302 EXPORT_SYMBOL(__downgrade_write);