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 */
14 void __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
29 EXPORT_SYMBOL(__init_rwsem);
30
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
37 };
38
39 /*
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
48 */
49 static inline struct rw_semaphore *
50 __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
51 {
52 struct rwsem_waiter *waiter;
53 struct task_struct *tsk;
54 struct list_head *next;
55 signed long oldcount, woken, loop;
56
57 if (downgrading)
58 goto dont_wake_writers;
59
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
62 */
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;
68
69 waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
70
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
74 */
75 if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
76 goto readers_only;
77
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).
81 */
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;
89
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;
95
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
100 */
101 readers_only:
102 woken = 0;
103 do {
104 woken++;
105
106 if (waiter->list.next == &sem->wait_list)
107 break;
108
109 waiter = list_entry(waiter->list.next,
110 struct rwsem_waiter, list);
111
112 } while (waiter->flags & RWSEM_WAITING_FOR_READ);
113
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;
119
120 rwsem_atomic_add(woken, sem);
121
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);
131 }
132
133 sem->wait_list.next = next;
134 next->prev = &sem->wait_list;
135
136 out:
137 return sem;
138
139 /* undo the change to count, but check for a transition 1->0 */
140 undo:
141 if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) != 0)
142 goto out;
143 goto try_again;
144 }
145
146 /*
147 * wait for a lock to be granted
148 */
149 static struct rw_semaphore __sched *
150 rwsem_down_failed_common(struct rw_semaphore *sem,
151 struct rwsem_waiter *waiter, signed long adjustment)
152 {
153 struct task_struct *tsk = current;
154 signed long count;
155
156 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
157
158 /* set up my own style of waitqueue */
159 spin_lock_irq(&sem->wait_lock);
160 waiter->task = tsk;
161 get_task_struct(tsk);
162
163 list_add_tail(&waiter->list, &sem->wait_list);
164
165 /* we're now waiting on the lock, but no longer actively read-locking */
166 count = rwsem_atomic_update(adjustment, sem);
167
168 /* if there are no active locks, wake the front queued process(es) up */
169 if (!(count & RWSEM_ACTIVE_MASK))
170 sem = __rwsem_do_wake(sem, 0);
171
172 spin_unlock_irq(&sem->wait_lock);
173
174 /* wait to be given the lock */
175 for (;;) {
176 if (!waiter->task)
177 break;
178 schedule();
179 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
180 }
181
182 tsk->state = TASK_RUNNING;
183
184 return sem;
185 }
186
187 /*
188 * wait for the read lock to be granted
189 */
190 asmregparm struct rw_semaphore __sched *
191 rwsem_down_read_failed(struct rw_semaphore *sem)
192 {
193 struct rwsem_waiter waiter;
194
195 waiter.flags = RWSEM_WAITING_FOR_READ;
196 rwsem_down_failed_common(sem, &waiter,
197 RWSEM_WAITING_BIAS - RWSEM_ACTIVE_BIAS);
198 return sem;
199 }
200
201 /*
202 * wait for the write lock to be granted
203 */
204 asmregparm struct rw_semaphore __sched *
205 rwsem_down_write_failed(struct rw_semaphore *sem)
206 {
207 struct rwsem_waiter waiter;
208
209 waiter.flags = RWSEM_WAITING_FOR_WRITE;
210 rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_BIAS);
211
212 return sem;
213 }
214
215 /*
216 * handle waking up a waiter on the semaphore
217 * - up_read/up_write has decremented the active part of count if we come here
218 */
219 asmregparm struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
220 {
221 unsigned long flags;
222
223 spin_lock_irqsave(&sem->wait_lock, flags);
224
225 /* do nothing if list empty */
226 if (!list_empty(&sem->wait_list))
227 sem = __rwsem_do_wake(sem, 0);
228
229 spin_unlock_irqrestore(&sem->wait_lock, flags);
230
231 return sem;
232 }
233
234 /*
235 * downgrade a write lock into a read lock
236 * - caller incremented waiting part of count and discovered it still negative
237 * - just wake up any readers at the front of the queue
238 */
239 asmregparm struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
240 {
241 unsigned long flags;
242
243 spin_lock_irqsave(&sem->wait_lock, flags);
244
245 /* do nothing if list empty */
246 if (!list_empty(&sem->wait_list))
247 sem = __rwsem_do_wake(sem, 1);
248
249 spin_unlock_irqrestore(&sem->wait_lock, flags);
250
251 return sem;
252 }
253
254 EXPORT_SYMBOL(rwsem_down_read_failed);
255 EXPORT_SYMBOL(rwsem_down_write_failed);
256 EXPORT_SYMBOL(rwsem_wake);
257 EXPORT_SYMBOL(rwsem_downgrade_wake);
258
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