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Linus Torvalds1da177e2005-04-16 15:20:36 -07001/* 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
Ingo Molnar4ea21762006-07-03 00:24:53 -070011/*
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));
Peter Zijlstra4dfbb9d2006-10-11 01:45:14 -040022 lockdep_init_map(&sem->dep_map, name, key, 0);
Ingo Molnar4ea21762006-07-03 00:24:53 -070023#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
Linus Torvalds1da177e2005-04-16 15:20:36 -070031struct 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
Linus Torvalds1da177e2005-04-16 15:20:36 -070039/*
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 */
49static 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
Linus Torvalds1da177e2005-04-16 15:20:36 -070057 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;
akpm@osdl.orgd59dd462005-05-01 08:58:47 -070084 smp_mb();
Linus Torvalds1da177e2005-04-16 15:20:36 -070085 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;
akpm@osdl.orgd59dd462005-05-01 08:58:47 -0700127 smp_mb();
Linus Torvalds1da177e2005-04-16 15:20:36 -0700128 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:
Linus Torvalds1da177e2005-04-16 15:20:36 -0700137 return sem;
138
Michel Lespinasse91af7082010-05-12 11:38:45 +0100139 /* undo the change to the active count, but check for a transition
140 * 1->0 */
Linus Torvalds1da177e2005-04-16 15:20:36 -0700141 undo:
Michel Lespinasse91af7082010-05-12 11:38:45 +0100142 if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700143 goto out;
144 goto try_again;
145}
146
147/*
148 * wait for a lock to be granted
149 */
Livio Soaresc7af77b2007-12-18 15:21:13 +0100150static struct rw_semaphore __sched *
Linus Torvalds1da177e2005-04-16 15:20:36 -0700151rwsem_down_failed_common(struct rw_semaphore *sem,
152 struct rwsem_waiter *waiter, signed long adjustment)
153{
154 struct task_struct *tsk = current;
155 signed long count;
156
157 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
158
159 /* set up my own style of waitqueue */
160 spin_lock_irq(&sem->wait_lock);
161 waiter->task = tsk;
162 get_task_struct(tsk);
163
164 list_add_tail(&waiter->list, &sem->wait_list);
165
166 /* we're now waiting on the lock, but no longer actively read-locking */
167 count = rwsem_atomic_update(adjustment, sem);
168
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);
172
173 spin_unlock_irq(&sem->wait_lock);
174
175 /* wait to be given the lock */
176 for (;;) {
177 if (!waiter->task)
178 break;
179 schedule();
180 set_task_state(tsk, TASK_UNINTERRUPTIBLE);
181 }
182
183 tsk->state = TASK_RUNNING;
184
185 return sem;
186}
187
188/*
189 * wait for the read lock to be granted
190 */
Ingo Molnard50efc62008-01-30 13:33:00 +0100191asmregparm struct rw_semaphore __sched *
Linus Torvalds1da177e2005-04-16 15:20:36 -0700192rwsem_down_read_failed(struct rw_semaphore *sem)
193{
194 struct rwsem_waiter waiter;
195
Linus Torvalds1da177e2005-04-16 15:20:36 -0700196 waiter.flags = RWSEM_WAITING_FOR_READ;
197 rwsem_down_failed_common(sem, &waiter,
198 RWSEM_WAITING_BIAS - RWSEM_ACTIVE_BIAS);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700199 return sem;
200}
201
202/*
203 * wait for the write lock to be granted
204 */
Ingo Molnard50efc62008-01-30 13:33:00 +0100205asmregparm struct rw_semaphore __sched *
Linus Torvalds1da177e2005-04-16 15:20:36 -0700206rwsem_down_write_failed(struct rw_semaphore *sem)
207{
208 struct rwsem_waiter waiter;
209
Linus Torvalds1da177e2005-04-16 15:20:36 -0700210 waiter.flags = RWSEM_WAITING_FOR_WRITE;
211 rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_BIAS);
212
Linus Torvalds1da177e2005-04-16 15:20:36 -0700213 return sem;
214}
215
216/*
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
219 */
Ingo Molnard50efc62008-01-30 13:33:00 +0100220asmregparm struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700221{
222 unsigned long flags;
223
Linus Torvalds1da177e2005-04-16 15:20:36 -0700224 spin_lock_irqsave(&sem->wait_lock, flags);
225
226 /* do nothing if list empty */
227 if (!list_empty(&sem->wait_list))
228 sem = __rwsem_do_wake(sem, 0);
229
230 spin_unlock_irqrestore(&sem->wait_lock, flags);
231
Linus Torvalds1da177e2005-04-16 15:20:36 -0700232 return sem;
233}
234
235/*
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
239 */
Ingo Molnard50efc62008-01-30 13:33:00 +0100240asmregparm struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700241{
242 unsigned long flags;
243
Linus Torvalds1da177e2005-04-16 15:20:36 -0700244 spin_lock_irqsave(&sem->wait_lock, flags);
245
246 /* do nothing if list empty */
247 if (!list_empty(&sem->wait_list))
248 sem = __rwsem_do_wake(sem, 1);
249
250 spin_unlock_irqrestore(&sem->wait_lock, flags);
251
Linus Torvalds1da177e2005-04-16 15:20:36 -0700252 return sem;
253}
254
255EXPORT_SYMBOL(rwsem_down_read_failed);
256EXPORT_SYMBOL(rwsem_down_write_failed);
257EXPORT_SYMBOL(rwsem_wake);
258EXPORT_SYMBOL(rwsem_downgrade_wake);