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1/* rwsem.c: R/W semaphores: contention handling functions
2 *
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>
10
11/*
12 * Initialize an rwsem:
13 */
14void __init_rwsem(struct rw_semaphore *sem, const char *name,
15 struct lock_class_key *key)
16{
17#ifdef CONFIG_DEBUG_LOCK_ALLOC
18 /*
19 * Make sure we are not reinitializing a held semaphore:
20 */
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);
27}
28
29EXPORT_SYMBOL(__init_rwsem);
30
31struct 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
37};
38
39/* Wake types for __rwsem_do_wake(). Note that RWSEM_WAKE_NO_ACTIVE and
40 * RWSEM_WAKE_READ_OWNED imply that the spinlock must have been kept held
41 * since the rwsem value was observed.
42 */
43#define RWSEM_WAKE_ANY 0 /* Wake whatever's at head of wait list */
44#define RWSEM_WAKE_NO_ACTIVE 1 /* rwsem was observed with no active thread */
45#define RWSEM_WAKE_READ_OWNED 2 /* rwsem was observed to be read owned */
46
47/*
48 * handle the lock release when processes blocked on it that can now run
49 * - if we come here from up_xxxx(), then:
50 * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
51 * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
52 * - there must be someone on the queue
53 * - the spinlock must be held by the caller
54 * - woken process blocks are discarded from the list after having task zeroed
55 * - writers are only woken if downgrading is false
56 */
57static struct rw_semaphore *
58__rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
59{
60 struct rwsem_waiter *waiter;
61 struct task_struct *tsk;
62 struct list_head *next;
63 signed long oldcount, woken, loop, adjustment;
64
65 waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
66 if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
67 goto readers_only;
68
69 if (wake_type == RWSEM_WAKE_READ_OWNED)
70 /* Another active reader was observed, so wakeup is not
71 * likely to succeed. Save the atomic op.
72 */
73 goto out;
74
75 /* There's a writer at the front of the queue - try to grant it the
76 * write lock. However, we only wake this writer if we can transition
77 * the active part of the count from 0 -> 1
78 */
79 adjustment = RWSEM_ACTIVE_WRITE_BIAS;
80 if (waiter->list.next == &sem->wait_list)
81 adjustment -= RWSEM_WAITING_BIAS;
82
83 try_again_write:
84 oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
85 if (oldcount & RWSEM_ACTIVE_MASK)
86 /* Someone grabbed the sem already */
87 goto undo_write;
88
89 /* We must be careful not to touch 'waiter' after we set ->task = NULL.
90 * It is an allocated on the waiter's stack and may become invalid at
91 * any time after that point (due to a wakeup from another source).
92 */
93 list_del(&waiter->list);
94 tsk = waiter->task;
95 smp_mb();
96 waiter->task = NULL;
97 wake_up_process(tsk);
98 put_task_struct(tsk);
99 goto out;
100
101 readers_only:
102 /* If we come here from up_xxxx(), another thread might have reached
103 * rwsem_down_failed_common() before we acquired the spinlock and
104 * woken up a waiter, making it now active. We prefer to check for
105 * this first in order to not spend too much time with the spinlock
106 * held if we're not going to be able to wake up readers in the end.
107 *
108 * Note that we do not need to update the rwsem count: any writer
109 * trying to acquire rwsem will run rwsem_down_write_failed() due
110 * to the waiting threads and block trying to acquire the spinlock.
111 *
112 * We use a dummy atomic update in order to acquire the cache line
113 * exclusively since we expect to succeed and run the final rwsem
114 * count adjustment pretty soon.
115 */
116 if (wake_type == RWSEM_WAKE_ANY &&
117 rwsem_atomic_update(0, sem) < RWSEM_WAITING_BIAS)
118 /* Someone grabbed the sem for write already */
119 goto out;
120
121 /* Grant an infinite number of read locks to the readers at the front
122 * of the queue. Note we increment the 'active part' of the count by
123 * the number of readers before waking any processes up.
124 */
125 woken = 0;
126 do {
127 woken++;
128
129 if (waiter->list.next == &sem->wait_list)
130 break;
131
132 waiter = list_entry(waiter->list.next,
133 struct rwsem_waiter, list);
134
135 } while (waiter->flags & RWSEM_WAITING_FOR_READ);
136
137 adjustment = woken * RWSEM_ACTIVE_READ_BIAS;
138 if (waiter->flags & RWSEM_WAITING_FOR_READ)
139 /* hit end of list above */
140 adjustment -= RWSEM_WAITING_BIAS;
141
142 rwsem_atomic_add(adjustment, sem);
143
144 next = sem->wait_list.next;
145 for (loop = woken; loop > 0; loop--) {
146 waiter = list_entry(next, struct rwsem_waiter, list);
147 next = waiter->list.next;
148 tsk = waiter->task;
149 smp_mb();
150 waiter->task = NULL;
151 wake_up_process(tsk);
152 put_task_struct(tsk);
153 }
154
155 sem->wait_list.next = next;
156 next->prev = &sem->wait_list;
157
158 out:
159 return sem;
160
161 /* undo the change to the active count, but check for a transition
162 * 1->0 */
163 undo_write:
164 if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK)
165 goto out;
166 goto try_again_write;
167}
168
169/*
170 * wait for a lock to be granted
171 */
172static struct rw_semaphore __sched *
173rwsem_down_failed_common(struct rw_semaphore *sem,
174 unsigned int flags, signed long adjustment)
175{
176 struct rwsem_waiter waiter;
177 struct task_struct *tsk = current;
178 signed long count;
179
180 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
181
182 /* set up my own style of waitqueue */
183 spin_lock_irq(&sem->wait_lock);
184 waiter.task = tsk;
185 waiter.flags = flags;
186 get_task_struct(tsk);
187
188 if (list_empty(&sem->wait_list))
189 adjustment += RWSEM_WAITING_BIAS;
190 list_add_tail(&waiter.list, &sem->wait_list);
191
192 /* we're now waiting on the lock, but no longer actively locking */
193 count = rwsem_atomic_update(adjustment, sem);
194
195 /* If there are no active locks, wake the front queued process(es) up.
196 *
197 * Alternatively, if we're called from a failed down_write(), there
198 * were already threads queued before us and there are no active
199 * writers, the lock must be read owned; so we try to wake any read
200 * locks that were queued ahead of us. */
201 if (count == RWSEM_WAITING_BIAS)
202 sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE);
203 else if (count > RWSEM_WAITING_BIAS &&
204 adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
205 sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
206
207 spin_unlock_irq(&sem->wait_lock);
208
209 /* wait to be given the lock */
210 for (;;) {
211 if (!waiter.task)
212 break;
213 schedule();
214 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
215 }
216
217 tsk->state = TASK_RUNNING;
218
219 return sem;
220}
221
222/*
223 * wait for the read lock to be granted
224 */
225struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
226{
227 return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_READ,
228 -RWSEM_ACTIVE_READ_BIAS);
229}
230
231/*
232 * wait for the write lock to be granted
233 */
234struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
235{
236 return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_WRITE,
237 -RWSEM_ACTIVE_WRITE_BIAS);
238}
239
240/*
241 * handle waking up a waiter on the semaphore
242 * - up_read/up_write has decremented the active part of count if we come here
243 */
244struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
245{
246 unsigned long flags;
247
248 spin_lock_irqsave(&sem->wait_lock, flags);
249
250 /* do nothing if list empty */
251 if (!list_empty(&sem->wait_list))
252 sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
253
254 spin_unlock_irqrestore(&sem->wait_lock, flags);
255
256 return sem;
257}
258
259/*
260 * downgrade a write lock into a read lock
261 * - caller incremented waiting part of count and discovered it still negative
262 * - just wake up any readers at the front of the queue
263 */
264struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
265{
266 unsigned long flags;
267
268 spin_lock_irqsave(&sem->wait_lock, flags);
269
270 /* do nothing if list empty */
271 if (!list_empty(&sem->wait_list))
272 sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
273
274 spin_unlock_irqrestore(&sem->wait_lock, flags);
275
276 return sem;
277}
278
279EXPORT_SYMBOL(rwsem_down_read_failed);
280EXPORT_SYMBOL(rwsem_down_write_failed);
281EXPORT_SYMBOL(rwsem_wake);
282EXPORT_SYMBOL(rwsem_downgrade_wake);