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1da177e4 LT |
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 | ||
4ea2176d IM |
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)); | |
4dfbb9d8 | 22 | lockdep_init_map(&sem->dep_map, name, key, 0); |
4ea2176d IM |
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 | ||
1da177e4 LT |
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 | ||
70bdc6e0 ML |
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 | ||
1da177e4 LT |
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) | |
345af7bf | 52 | * - there must be someone on the queue |
1da177e4 LT |
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 | */ | |
70bdc6e0 ML |
57 | static struct rw_semaphore * |
58 | __rwsem_do_wake(struct rw_semaphore *sem, int wake_type) | |
1da177e4 LT |
59 | { |
60 | struct rwsem_waiter *waiter; | |
61 | struct task_struct *tsk; | |
62 | struct list_head *next; | |
63 | signed long oldcount, woken, loop; | |
64 | ||
345af7bf ML |
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 | ||
70bdc6e0 | 69 | if (wake_type == RWSEM_WAKE_READ_OWNED) |
345af7bf | 70 | goto out; |
1da177e4 | 71 | |
345af7bf ML |
72 | /* There's a writer at the front of the queue - try to grant it the |
73 | * write lock. However, we only wake this writer if we can transition | |
74 | * the active part of the count from 0 -> 1 | |
1da177e4 | 75 | */ |
345af7bf | 76 | try_again_write: |
1da177e4 LT |
77 | oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem) |
78 | - RWSEM_ACTIVE_BIAS; | |
79 | if (oldcount & RWSEM_ACTIVE_MASK) | |
345af7bf ML |
80 | /* Someone grabbed the sem already */ |
81 | goto undo_write; | |
1da177e4 LT |
82 | |
83 | /* We must be careful not to touch 'waiter' after we set ->task = NULL. | |
84 | * It is an allocated on the waiter's stack and may become invalid at | |
85 | * any time after that point (due to a wakeup from another source). | |
86 | */ | |
87 | list_del(&waiter->list); | |
88 | tsk = waiter->task; | |
d59dd462 | 89 | smp_mb(); |
1da177e4 LT |
90 | waiter->task = NULL; |
91 | wake_up_process(tsk); | |
92 | put_task_struct(tsk); | |
93 | goto out; | |
94 | ||
345af7bf | 95 | readers_only: |
70bdc6e0 ML |
96 | /* If we come here from up_xxxx(), another thread might have reached |
97 | * rwsem_down_failed_common() before we acquired the spinlock and | |
98 | * woken up a waiter, making it now active. We prefer to check for | |
99 | * this first in order to not spend too much time with the spinlock | |
100 | * held if we're not going to be able to wake up readers in the end. | |
101 | * | |
102 | * Note that we do not need to update the rwsem count: any writer | |
103 | * trying to acquire rwsem will run rwsem_down_write_failed() due | |
104 | * to the waiting threads and block trying to acquire the spinlock. | |
105 | * | |
106 | * We use a dummy atomic update in order to acquire the cache line | |
107 | * exclusively since we expect to succeed and run the final rwsem | |
108 | * count adjustment pretty soon. | |
109 | */ | |
110 | if (wake_type == RWSEM_WAKE_ANY && | |
111 | (rwsem_atomic_update(0, sem) & RWSEM_ACTIVE_MASK)) | |
345af7bf | 112 | /* Someone grabbed the sem already */ |
70bdc6e0 | 113 | goto out; |
1da177e4 | 114 | |
345af7bf ML |
115 | /* Grant an infinite number of read locks to the readers at the front |
116 | * of the queue. Note we increment the 'active part' of the count by | |
117 | * the number of readers before waking any processes up. | |
1da177e4 | 118 | */ |
1da177e4 LT |
119 | woken = 0; |
120 | do { | |
121 | woken++; | |
122 | ||
123 | if (waiter->list.next == &sem->wait_list) | |
124 | break; | |
125 | ||
126 | waiter = list_entry(waiter->list.next, | |
127 | struct rwsem_waiter, list); | |
128 | ||
129 | } while (waiter->flags & RWSEM_WAITING_FOR_READ); | |
130 | ||
131 | loop = woken; | |
132 | woken *= RWSEM_ACTIVE_BIAS - RWSEM_WAITING_BIAS; | |
1da177e4 LT |
133 | |
134 | rwsem_atomic_add(woken, sem); | |
135 | ||
136 | next = sem->wait_list.next; | |
137 | for (; loop > 0; loop--) { | |
138 | waiter = list_entry(next, struct rwsem_waiter, list); | |
139 | next = waiter->list.next; | |
140 | tsk = waiter->task; | |
d59dd462 | 141 | smp_mb(); |
1da177e4 LT |
142 | waiter->task = NULL; |
143 | wake_up_process(tsk); | |
144 | put_task_struct(tsk); | |
145 | } | |
146 | ||
147 | sem->wait_list.next = next; | |
148 | next->prev = &sem->wait_list; | |
149 | ||
150 | out: | |
1da177e4 LT |
151 | return sem; |
152 | ||
91af7081 ML |
153 | /* undo the change to the active count, but check for a transition |
154 | * 1->0 */ | |
345af7bf ML |
155 | undo_write: |
156 | if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK) | |
157 | goto out; | |
158 | goto try_again_write; | |
1da177e4 LT |
159 | } |
160 | ||
161 | /* | |
162 | * wait for a lock to be granted | |
163 | */ | |
c7af77b5 | 164 | static struct rw_semaphore __sched * |
1da177e4 LT |
165 | rwsem_down_failed_common(struct rw_semaphore *sem, |
166 | struct rwsem_waiter *waiter, signed long adjustment) | |
167 | { | |
168 | struct task_struct *tsk = current; | |
169 | signed long count; | |
170 | ||
171 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); | |
172 | ||
173 | /* set up my own style of waitqueue */ | |
174 | spin_lock_irq(&sem->wait_lock); | |
175 | waiter->task = tsk; | |
176 | get_task_struct(tsk); | |
177 | ||
178 | list_add_tail(&waiter->list, &sem->wait_list); | |
179 | ||
70bdc6e0 | 180 | /* we're now waiting on the lock, but no longer actively locking */ |
1da177e4 LT |
181 | count = rwsem_atomic_update(adjustment, sem); |
182 | ||
183 | /* if there are no active locks, wake the front queued process(es) up */ | |
184 | if (!(count & RWSEM_ACTIVE_MASK)) | |
70bdc6e0 | 185 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE); |
1da177e4 LT |
186 | |
187 | spin_unlock_irq(&sem->wait_lock); | |
188 | ||
189 | /* wait to be given the lock */ | |
190 | for (;;) { | |
191 | if (!waiter->task) | |
192 | break; | |
193 | schedule(); | |
194 | set_task_state(tsk, TASK_UNINTERRUPTIBLE); | |
195 | } | |
196 | ||
197 | tsk->state = TASK_RUNNING; | |
198 | ||
199 | return sem; | |
200 | } | |
201 | ||
202 | /* | |
203 | * wait for the read lock to be granted | |
204 | */ | |
d50efc6c | 205 | asmregparm struct rw_semaphore __sched * |
1da177e4 LT |
206 | rwsem_down_read_failed(struct rw_semaphore *sem) |
207 | { | |
208 | struct rwsem_waiter waiter; | |
209 | ||
1da177e4 LT |
210 | waiter.flags = RWSEM_WAITING_FOR_READ; |
211 | rwsem_down_failed_common(sem, &waiter, | |
212 | RWSEM_WAITING_BIAS - RWSEM_ACTIVE_BIAS); | |
1da177e4 LT |
213 | return sem; |
214 | } | |
215 | ||
216 | /* | |
217 | * wait for the write lock to be granted | |
218 | */ | |
d50efc6c | 219 | asmregparm struct rw_semaphore __sched * |
1da177e4 LT |
220 | rwsem_down_write_failed(struct rw_semaphore *sem) |
221 | { | |
222 | struct rwsem_waiter waiter; | |
223 | ||
1da177e4 LT |
224 | waiter.flags = RWSEM_WAITING_FOR_WRITE; |
225 | rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_BIAS); | |
226 | ||
1da177e4 LT |
227 | return sem; |
228 | } | |
229 | ||
230 | /* | |
231 | * handle waking up a waiter on the semaphore | |
232 | * - up_read/up_write has decremented the active part of count if we come here | |
233 | */ | |
d50efc6c | 234 | asmregparm struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) |
1da177e4 LT |
235 | { |
236 | unsigned long flags; | |
237 | ||
1da177e4 LT |
238 | spin_lock_irqsave(&sem->wait_lock, flags); |
239 | ||
240 | /* do nothing if list empty */ | |
241 | if (!list_empty(&sem->wait_list)) | |
70bdc6e0 | 242 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY); |
1da177e4 LT |
243 | |
244 | spin_unlock_irqrestore(&sem->wait_lock, flags); | |
245 | ||
1da177e4 LT |
246 | return sem; |
247 | } | |
248 | ||
249 | /* | |
250 | * downgrade a write lock into a read lock | |
251 | * - caller incremented waiting part of count and discovered it still negative | |
252 | * - just wake up any readers at the front of the queue | |
253 | */ | |
d50efc6c | 254 | asmregparm struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem) |
1da177e4 LT |
255 | { |
256 | unsigned long flags; | |
257 | ||
1da177e4 LT |
258 | spin_lock_irqsave(&sem->wait_lock, flags); |
259 | ||
260 | /* do nothing if list empty */ | |
261 | if (!list_empty(&sem->wait_list)) | |
70bdc6e0 | 262 | sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED); |
1da177e4 LT |
263 | |
264 | spin_unlock_irqrestore(&sem->wait_lock, flags); | |
265 | ||
1da177e4 LT |
266 | return sem; |
267 | } | |
268 | ||
269 | EXPORT_SYMBOL(rwsem_down_read_failed); | |
270 | EXPORT_SYMBOL(rwsem_down_write_failed); | |
271 | EXPORT_SYMBOL(rwsem_wake); | |
272 | EXPORT_SYMBOL(rwsem_downgrade_wake); |