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 void rwsemtrace(struct rw_semaphore *sem, const char *str)
24 printk("[%d] %s({%d,%d})\n",
25 current->pid, str, sem->activity,
26 list_empty(&sem->wait_list) ? 0 : 1);
31 * initialise the semaphore
33 void fastcall init_rwsem(struct rw_semaphore *sem)
36 spin_lock_init(&sem->wait_lock);
37 INIT_LIST_HEAD(&sem->wait_list);
44 * handle the lock release when processes blocked on it that can now run
45 * - if we come here, then:
46 * - the 'active count' _reached_ zero
47 * - the 'waiting count' is non-zero
48 * - the spinlock must be held by the caller
49 * - woken process blocks are discarded from the list after having task zeroed
50 * - writers are only woken if wakewrite is non-zero
52 static inline struct rw_semaphore *
53 __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
55 struct rwsem_waiter *waiter;
56 struct task_struct *tsk;
59 rwsemtrace(sem, "Entering __rwsem_do_wake");
61 waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
64 if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
66 goto dont_wake_writers;
69 /* if we are allowed to wake writers try to grant a single write lock
70 * if there's a writer at the front of the queue
71 * - we leave the 'waiting count' incremented to signify potential
74 if (waiter->flags & RWSEM_WAITING_FOR_WRITE) {
76 list_del(&waiter->list);
78 /* Don't touch waiter after ->task has been NULLed */
86 /* grant an infinite number of read locks to the front of the queue */
89 while (waiter->flags & RWSEM_WAITING_FOR_READ) {
90 struct list_head *next = waiter->list.next;
92 list_del(&waiter->list);
99 if (list_empty(&sem->wait_list))
101 waiter = list_entry(next, struct rwsem_waiter, list);
104 sem->activity += woken;
107 rwsemtrace(sem, "Leaving __rwsem_do_wake");
112 * wake a single writer
114 static inline struct rw_semaphore *
115 __rwsem_wake_one_writer(struct rw_semaphore *sem)
117 struct rwsem_waiter *waiter;
118 struct task_struct *tsk;
122 waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
123 list_del(&waiter->list);
128 wake_up_process(tsk);
129 put_task_struct(tsk);
134 * get a read lock on the semaphore
136 void fastcall __sched __down_read(struct rw_semaphore *sem)
138 struct rwsem_waiter waiter;
139 struct task_struct *tsk;
141 rwsemtrace(sem, "Entering __down_read");
143 spin_lock(&sem->wait_lock);
145 if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
148 spin_unlock(&sem->wait_lock);
153 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
155 /* set up my own style of waitqueue */
157 waiter.flags = RWSEM_WAITING_FOR_READ;
158 get_task_struct(tsk);
160 list_add_tail(&waiter.list, &sem->wait_list);
162 /* we don't need to touch the semaphore struct anymore */
163 spin_unlock(&sem->wait_lock);
165 /* wait to be given the lock */
170 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
173 tsk->state = TASK_RUNNING;
176 rwsemtrace(sem, "Leaving __down_read");
180 * trylock for reading -- returns 1 if successful, 0 if contention
182 int fastcall __down_read_trylock(struct rw_semaphore *sem)
185 rwsemtrace(sem, "Entering __down_read_trylock");
187 spin_lock(&sem->wait_lock);
189 if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
195 spin_unlock(&sem->wait_lock);
197 rwsemtrace(sem, "Leaving __down_read_trylock");
202 * get a write lock on the semaphore
203 * - we increment the waiting count anyway to indicate an exclusive lock
205 void fastcall __sched __down_write(struct rw_semaphore *sem)
207 struct rwsem_waiter waiter;
208 struct task_struct *tsk;
210 rwsemtrace(sem, "Entering __down_write");
212 spin_lock(&sem->wait_lock);
214 if (sem->activity == 0 && list_empty(&sem->wait_list)) {
217 spin_unlock(&sem->wait_lock);
222 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
224 /* set up my own style of waitqueue */
226 waiter.flags = RWSEM_WAITING_FOR_WRITE;
227 get_task_struct(tsk);
229 list_add_tail(&waiter.list, &sem->wait_list);
231 /* we don't need to touch the semaphore struct anymore */
232 spin_unlock(&sem->wait_lock);
234 /* wait to be given the lock */
239 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
242 tsk->state = TASK_RUNNING;
245 rwsemtrace(sem, "Leaving __down_write");
249 * trylock for writing -- returns 1 if successful, 0 if contention
251 int fastcall __down_write_trylock(struct rw_semaphore *sem)
254 rwsemtrace(sem, "Entering __down_write_trylock");
256 spin_lock(&sem->wait_lock);
258 if (sem->activity == 0 && list_empty(&sem->wait_list)) {
264 spin_unlock(&sem->wait_lock);
266 rwsemtrace(sem, "Leaving __down_write_trylock");
271 * release a read lock on the semaphore
273 void fastcall __up_read(struct rw_semaphore *sem)
275 rwsemtrace(sem, "Entering __up_read");
277 spin_lock(&sem->wait_lock);
279 if (--sem->activity == 0 && !list_empty(&sem->wait_list))
280 sem = __rwsem_wake_one_writer(sem);
282 spin_unlock(&sem->wait_lock);
284 rwsemtrace(sem, "Leaving __up_read");
288 * release a write lock on the semaphore
290 void fastcall __up_write(struct rw_semaphore *sem)
292 rwsemtrace(sem, "Entering __up_write");
294 spin_lock(&sem->wait_lock);
297 if (!list_empty(&sem->wait_list))
298 sem = __rwsem_do_wake(sem, 1);
300 spin_unlock(&sem->wait_lock);
302 rwsemtrace(sem, "Leaving __up_write");
306 * downgrade a write lock into a read lock
307 * - just wake up any readers at the front of the queue
309 void fastcall __downgrade_write(struct rw_semaphore *sem)
311 rwsemtrace(sem, "Entering __downgrade_write");
313 spin_lock(&sem->wait_lock);
316 if (!list_empty(&sem->wait_list))
317 sem = __rwsem_do_wake(sem, 0);
319 spin_unlock(&sem->wait_lock);
321 rwsemtrace(sem, "Leaving __downgrade_write");
324 EXPORT_SYMBOL(init_rwsem);
325 EXPORT_SYMBOL(__down_read);
326 EXPORT_SYMBOL(__down_read_trylock);
327 EXPORT_SYMBOL(__down_write);
328 EXPORT_SYMBOL(__down_write_trylock);
329 EXPORT_SYMBOL(__up_read);
330 EXPORT_SYMBOL(__up_write);
331 EXPORT_SYMBOL(__downgrade_write);
333 EXPORT_SYMBOL(rwsemtrace);