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/*
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
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 */
149static struct rw_semaphore *
150rwsem_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 */
190struct rw_semaphore fastcall __sched *
191rwsem_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 */
204struct rw_semaphore fastcall __sched *
205rwsem_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 */
219struct rw_semaphore fastcall *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 */
239struct rw_semaphore fastcall *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
254EXPORT_SYMBOL(rwsem_down_read_failed);
255EXPORT_SYMBOL(rwsem_down_write_failed);
256EXPORT_SYMBOL(rwsem_wake);
257EXPORT_SYMBOL(rwsem_downgrade_wake);
258