Driver-core: Always create class directories for classses that support namespaces.
[linux-2.6/linux-acpi-2.6/ibm-acpi-2.6.git] / lib / rwsem.c
blobceba8e28807a81c09c551e4f782091efa9bebccf
1 /* rwsem.c: R/W semaphores: contention handling functions
3 * Written by David Howells (dhowells@redhat.com).
4 * Derived from arch/i386/kernel/semaphore.c
5 */
6 #include <linux/rwsem.h>
7 #include <linux/sched.h>
8 #include <linux/init.h>
9 #include <linux/module.h>
12 * Initialize an rwsem:
14 void __init_rwsem(struct rw_semaphore *sem, const char *name,
15 struct lock_class_key *key)
17 #ifdef CONFIG_DEBUG_LOCK_ALLOC
19 * Make sure we are not reinitializing a held semaphore:
21 debug_check_no_locks_freed((void *)sem, sizeof(*sem));
22 lockdep_init_map(&sem->dep_map, name, key, 0);
23 #endif
24 sem->count = RWSEM_UNLOCKED_VALUE;
25 spin_lock_init(&sem->wait_lock);
26 INIT_LIST_HEAD(&sem->wait_list);
29 EXPORT_SYMBOL(__init_rwsem);
31 struct rwsem_waiter {
32 struct list_head list;
33 struct task_struct *task;
34 unsigned int flags;
35 #define RWSEM_WAITING_FOR_READ 0x00000001
36 #define RWSEM_WAITING_FOR_WRITE 0x00000002
40 * handle the lock release when processes blocked on it that can now run
41 * - if we come here from up_xxxx(), then:
42 * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
43 * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
44 * - there must be someone on the queue
45 * - the spinlock must be held by the caller
46 * - woken process blocks are discarded from the list after having task zeroed
47 * - writers are only woken if downgrading is false
49 static inline struct rw_semaphore *
50 __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
52 struct rwsem_waiter *waiter;
53 struct task_struct *tsk;
54 struct list_head *next;
55 signed long oldcount, woken, loop;
57 if (downgrading)
58 goto dont_wake_writers;
60 /* if we came through an up_xxxx() call, we only only wake someone up
61 * if we can transition the active part of the count from 0 -> 1
63 try_again:
64 oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
65 - RWSEM_ACTIVE_BIAS;
66 if (oldcount & RWSEM_ACTIVE_MASK)
67 goto undo;
69 waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
71 /* try to grant a single write lock if there's a writer at the front
72 * of the queue - note we leave the 'active part' of the count
73 * incremented by 1 and the waiting part incremented by 0x00010000
75 if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
76 goto readers_only;
78 /* We must be careful not to touch 'waiter' after we set ->task = NULL.
79 * It is an allocated on the waiter's stack and may become invalid at
80 * any time after that point (due to a wakeup from another source).
82 list_del(&waiter->list);
83 tsk = waiter->task;
84 smp_mb();
85 waiter->task = NULL;
86 wake_up_process(tsk);
87 put_task_struct(tsk);
88 goto out;
90 /* don't want to wake any writers */
91 dont_wake_writers:
92 waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
93 if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
94 goto out;
96 /* grant an infinite number of read locks to the readers at the front
97 * of the queue
98 * - note we increment the 'active part' of the count by the number of
99 * readers before waking any processes up
101 readers_only:
102 woken = 0;
103 do {
104 woken++;
106 if (waiter->list.next == &sem->wait_list)
107 break;
109 waiter = list_entry(waiter->list.next,
110 struct rwsem_waiter, list);
112 } while (waiter->flags & RWSEM_WAITING_FOR_READ);
114 loop = woken;
115 woken *= RWSEM_ACTIVE_BIAS - RWSEM_WAITING_BIAS;
116 if (!downgrading)
117 /* we'd already done one increment earlier */
118 woken -= RWSEM_ACTIVE_BIAS;
120 rwsem_atomic_add(woken, sem);
122 next = sem->wait_list.next;
123 for (; loop > 0; loop--) {
124 waiter = list_entry(next, struct rwsem_waiter, list);
125 next = waiter->list.next;
126 tsk = waiter->task;
127 smp_mb();
128 waiter->task = NULL;
129 wake_up_process(tsk);
130 put_task_struct(tsk);
133 sem->wait_list.next = next;
134 next->prev = &sem->wait_list;
136 out:
137 return sem;
139 /* undo the change to the active count, but check for a transition
140 * 1->0 */
141 undo:
142 if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK)
143 goto out;
144 goto try_again;
148 * wait for a lock to be granted
150 static struct rw_semaphore __sched *
151 rwsem_down_failed_common(struct rw_semaphore *sem,
152 struct rwsem_waiter *waiter, signed long adjustment)
154 struct task_struct *tsk = current;
155 signed long count;
157 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
159 /* set up my own style of waitqueue */
160 spin_lock_irq(&sem->wait_lock);
161 waiter->task = tsk;
162 get_task_struct(tsk);
164 list_add_tail(&waiter->list, &sem->wait_list);
166 /* we're now waiting on the lock, but no longer actively read-locking */
167 count = rwsem_atomic_update(adjustment, sem);
169 /* if there are no active locks, wake the front queued process(es) up */
170 if (!(count & RWSEM_ACTIVE_MASK))
171 sem = __rwsem_do_wake(sem, 0);
173 spin_unlock_irq(&sem->wait_lock);
175 /* wait to be given the lock */
176 for (;;) {
177 if (!waiter->task)
178 break;
179 schedule();
180 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
183 tsk->state = TASK_RUNNING;
185 return sem;
189 * wait for the read lock to be granted
191 asmregparm struct rw_semaphore __sched *
192 rwsem_down_read_failed(struct rw_semaphore *sem)
194 struct rwsem_waiter waiter;
196 waiter.flags = RWSEM_WAITING_FOR_READ;
197 rwsem_down_failed_common(sem, &waiter,
198 RWSEM_WAITING_BIAS - RWSEM_ACTIVE_BIAS);
199 return sem;
203 * wait for the write lock to be granted
205 asmregparm struct rw_semaphore __sched *
206 rwsem_down_write_failed(struct rw_semaphore *sem)
208 struct rwsem_waiter waiter;
210 waiter.flags = RWSEM_WAITING_FOR_WRITE;
211 rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_BIAS);
213 return sem;
217 * handle waking up a waiter on the semaphore
218 * - up_read/up_write has decremented the active part of count if we come here
220 asmregparm struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
222 unsigned long flags;
224 spin_lock_irqsave(&sem->wait_lock, flags);
226 /* do nothing if list empty */
227 if (!list_empty(&sem->wait_list))
228 sem = __rwsem_do_wake(sem, 0);
230 spin_unlock_irqrestore(&sem->wait_lock, flags);
232 return sem;
236 * downgrade a write lock into a read lock
237 * - caller incremented waiting part of count and discovered it still negative
238 * - just wake up any readers at the front of the queue
240 asmregparm struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
242 unsigned long flags;
244 spin_lock_irqsave(&sem->wait_lock, flags);
246 /* do nothing if list empty */
247 if (!list_empty(&sem->wait_list))
248 sem = __rwsem_do_wake(sem, 1);
250 spin_unlock_irqrestore(&sem->wait_lock, flags);
252 return sem;
255 EXPORT_SYMBOL(rwsem_down_read_failed);
256 EXPORT_SYMBOL(rwsem_down_write_failed);
257 EXPORT_SYMBOL(rwsem_wake);
258 EXPORT_SYMBOL(rwsem_downgrade_wake);