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1 jermar 1
/*
2
 * Copyright (C) 2001-2004 Jakub Jermar
3
 * All rights reserved.
4
 *
5
 * Redistribution and use in source and binary forms, with or without
6
 * modification, are permitted provided that the following conditions
7
 * are met:
8
 *
9
 * - Redistributions of source code must retain the above copyright
10
 *   notice, this list of conditions and the following disclaimer.
11
 * - Redistributions in binary form must reproduce the above copyright
12
 *   notice, this list of conditions and the following disclaimer in the
13
 *   documentation and/or other materials provided with the distribution.
14
 * - The name of the author may not be used to endorse or promote products
15
 *   derived from this software without specific prior written permission.
16
 *
17
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
18
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
19
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
20
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
21
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
22
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
23
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
24
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
25
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
26
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
27
 */
28
 
1757 jermar 29
/** @addtogroup sync
1702 cejka 30
 * @{
31
 */
32
 
1248 jermar 33
/**
1702 cejka 34
 * @file
1248 jermar 35
 * @brief   Reader/Writer locks.
557 jermar 36
 *
37
 * A reader/writer lock can be held by multiple readers at a time.
38
 * Or it can be exclusively held by a sole writer at a time.
1248 jermar 39
 *
1 jermar 40
 * These locks are not recursive.
1757 jermar 41
 * Because a technique called direct hand-off is used and because
42
 * waiting takes place in a single wait queue, neither readers
1248 jermar 43
 * nor writers will suffer starvation.
1 jermar 44
 *
45
 * If there is a writer followed by a reader waiting for the rwlock
46
 * and the writer times out, all leading readers are automatically woken up
47
 * and allowed in.
48
 */
49
 
50
/*
51
 * NOTE ON rwlock_holder_type
52
 * This field is set on an attempt to acquire the exclusive mutex
53
 * to the respective value depending whether the caller is a reader
54
 * or a writer. The field is examined only if the thread had been
55
 * previously blocked on the exclusive mutex. Thus it is save
56
 * to store the rwlock type in the thread structure, because
57
 * each thread can block on only one rwlock at a time.
58
 */
59
 
60
#include <synch/rwlock.h>
61
#include <synch/spinlock.h>
62
#include <synch/mutex.h>
63
#include <synch/waitq.h>
385 jermar 64
#include <synch/synch.h>
788 jermar 65
#include <adt/list.h>
1 jermar 66
#include <typedefs.h>
67
#include <arch/asm.h>
68
#include <arch.h>
69
#include <proc/thread.h>
70
#include <panic.h>
71
 
72
#define ALLOW_ALL       0
73
#define ALLOW_READERS_ONLY  1
74
 
75
static void let_others_in(rwlock_t *rwl, int readers_only);
76
static void release_spinlock(void *arg);
77
 
385 jermar 78
/** Initialize reader/writer lock
79
 *
80
 * Initialize reader/writer lock.
81
 *
82
 * @param rwl Reader/Writer lock.
83
 */
1 jermar 84
void rwlock_initialize(rwlock_t *rwl) {
557 jermar 85
    spinlock_initialize(&rwl->lock, "rwlock_t");
1 jermar 86
    mutex_initialize(&rwl->exclusive);
87
    rwl->readers_in = 0;
88
}
89
 
385 jermar 90
/** Acquire reader/writer lock for reading
91
 *
92
 * Acquire reader/writer lock for reading.
93
 * Timeout and willingness to block may be specified.
94
 *
95
 * @param rwl Reader/Writer lock.
96
 * @param usec Timeout in microseconds.
1502 jermar 97
 * @param flags Specify mode of operation.
385 jermar 98
 *
99
 * For exact description of possible combinations of
1502 jermar 100
 * usec and flags, see comment for waitq_sleep_timeout().
385 jermar 101
 *
102
 * @return See comment for waitq_sleep_timeout().
103
 */
1780 jermar 104
int _rwlock_write_lock_timeout(rwlock_t *rwl, uint32_t usec, int flags)
1 jermar 105
{
413 jermar 106
    ipl_t ipl;
1 jermar 107
    int rc;
108
 
413 jermar 109
    ipl = interrupts_disable();
15 jermar 110
    spinlock_lock(&THREAD->lock);
111
    THREAD->rwlock_holder_type = RWLOCK_WRITER;
112
    spinlock_unlock(&THREAD->lock);
413 jermar 113
    interrupts_restore(ipl);
1 jermar 114
 
115
    /*
116
     * Writers take the easy part.
117
     * They just need to acquire the exclusive mutex.
118
     */
1502 jermar 119
    rc = _mutex_lock_timeout(&rwl->exclusive, usec, flags);
1 jermar 120
    if (SYNCH_FAILED(rc)) {
121
 
122
        /*
1502 jermar 123
         * Lock operation timed out or was interrupted.
1 jermar 124
         * The state of rwl is UNKNOWN at this point.
125
         * No claims about its holder can be made.
126
         */
127
 
413 jermar 128
        ipl = interrupts_disable();
1 jermar 129
        spinlock_lock(&rwl->lock);
130
        /*
131
         * Now when rwl is locked, we can inspect it again.
132
         * If it is held by some readers already, we can let
133
         * readers from the head of the wait queue in.
134
         */
135
        if (rwl->readers_in)
136
            let_others_in(rwl, ALLOW_READERS_ONLY);
137
        spinlock_unlock(&rwl->lock);
413 jermar 138
        interrupts_restore(ipl);
1 jermar 139
    }
140
 
141
    return rc;
142
}
143
 
385 jermar 144
/** Acquire reader/writer lock for writing
145
 *
146
 * Acquire reader/writer lock for writing.
147
 * Timeout and willingness to block may be specified.
148
 *
149
 * @param rwl Reader/Writer lock.
150
 * @param usec Timeout in microseconds.
1502 jermar 151
 * @param flags Select mode of operation.
385 jermar 152
 *
153
 * For exact description of possible combinations of
1502 jermar 154
 * usec and flags, see comment for waitq_sleep_timeout().
385 jermar 155
 *
156
 * @return See comment for waitq_sleep_timeout().
157
 */
1780 jermar 158
int _rwlock_read_lock_timeout(rwlock_t *rwl, uint32_t usec, int flags)
1 jermar 159
{
160
    int rc;
413 jermar 161
    ipl_t ipl;
1 jermar 162
 
413 jermar 163
    ipl = interrupts_disable();
15 jermar 164
    spinlock_lock(&THREAD->lock);
165
    THREAD->rwlock_holder_type = RWLOCK_READER;
166
    spinlock_unlock(&THREAD->lock);
1 jermar 167
 
168
    spinlock_lock(&rwl->lock);
169
 
170
    /*
171
     * Find out whether we can get what we want without blocking.
172
     */
173
    rc = mutex_trylock(&rwl->exclusive);
174
    if (SYNCH_FAILED(rc)) {
175
 
176
        /*
177
         * 'exclusive' mutex is being held by someone else.
178
         * If the holder is a reader and there is no one
179
         * else waiting for it, we can enter the critical
180
         * section.
181
         */
182
 
183
        if (rwl->readers_in) {
184
            spinlock_lock(&rwl->exclusive.sem.wq.lock);
185
            if (list_empty(&rwl->exclusive.sem.wq.head)) {
186
                /*
187
                 * We can enter.
188
                 */
189
                spinlock_unlock(&rwl->exclusive.sem.wq.lock);
190
                goto shortcut;
191
            }
192
            spinlock_unlock(&rwl->exclusive.sem.wq.lock);
193
        }
194
 
195
        /*
196
         * In order to prevent a race condition when a reader
197
         * could block another reader at the head of the waitq,
198
         * we register a function to unlock rwl->lock
199
         * after this thread is put asleep.
200
         */
623 jermar 201
        #ifdef CONFIG_SMP
1 jermar 202
        thread_register_call_me(release_spinlock, &rwl->lock);
623 jermar 203
        #else
204
        thread_register_call_me(release_spinlock, NULL);
205
        #endif
1 jermar 206
 
1502 jermar 207
        rc = _mutex_lock_timeout(&rwl->exclusive, usec, flags);
1 jermar 208
        switch (rc) {
1888 jermar 209
        case ESYNCH_WOULD_BLOCK:
210
            /*
211
             * release_spinlock() wasn't called
212
             */
213
            thread_register_call_me(NULL, NULL);
214
            spinlock_unlock(&rwl->lock);
215
        case ESYNCH_TIMEOUT:
216
        case ESYNCH_INTERRUPTED:
217
            /*
218
             * The sleep timed out.
219
             * We just restore interrupt priority level.
220
             */
221
        case ESYNCH_OK_BLOCKED:    
222
            /*
223
             * We were woken with rwl->readers_in already incremented.
224
             * Note that this arrangement avoids race condition between
225
             * two concurrent readers. (Race is avoided if 'exclusive' is
226
             * locked at the same time as 'readers_in' is incremented.
227
             * Same time means both events happen atomically when
228
             * rwl->lock is held.)
229
             */
230
            interrupts_restore(ipl);
231
            break;
232
        case ESYNCH_OK_ATOMIC:
233
            panic("_mutex_lock_timeout()==ESYNCH_OK_ATOMIC\n");
234
            break;
235
        default:
236
            panic("invalid ESYNCH\n");
237
            break;
1 jermar 238
        }
239
        return rc;
240
    }
241
 
242
shortcut:
243
 
244
    /*
245
     * We can increment readers_in only if we didn't go to sleep.
246
     * For sleepers, rwlock_let_others_in() will do the job.
247
     */
248
    rwl->readers_in++;
249
 
250
    spinlock_unlock(&rwl->lock);
413 jermar 251
    interrupts_restore(ipl);
1 jermar 252
 
253
    return ESYNCH_OK_ATOMIC;
254
}
255
 
385 jermar 256
/** Release reader/writer lock held by writer
257
 *
258
 * Release reader/writer lock held by writer.
259
 * Handoff reader/writer lock ownership directly
260
 * to waiting readers or a writer.
261
 *
262
 * @param rwl Reader/Writer lock.
263
 */
1 jermar 264
void rwlock_write_unlock(rwlock_t *rwl)
265
{
413 jermar 266
    ipl_t ipl;
1 jermar 267
 
413 jermar 268
    ipl = interrupts_disable();
1 jermar 269
    spinlock_lock(&rwl->lock);
270
    let_others_in(rwl, ALLOW_ALL);
271
    spinlock_unlock(&rwl->lock);
413 jermar 272
    interrupts_restore(ipl);
1 jermar 273
 
274
}
275
 
385 jermar 276
/** Release reader/writer lock held by reader
277
 *
278
 * Release reader/writer lock held by reader.
279
 * Handoff reader/writer lock ownership directly
280
 * to a waiting writer or don't do anything if more
281
 * readers poses the lock.
282
 *
283
 * @param rwl Reader/Writer lock.
284
 */
1 jermar 285
void rwlock_read_unlock(rwlock_t *rwl)
286
{
413 jermar 287
    ipl_t ipl;
1 jermar 288
 
413 jermar 289
    ipl = interrupts_disable();
1 jermar 290
    spinlock_lock(&rwl->lock);
291
    if (!--rwl->readers_in)
292
        let_others_in(rwl, ALLOW_ALL);
293
    spinlock_unlock(&rwl->lock);
413 jermar 294
    interrupts_restore(ipl);
1 jermar 295
}
296
 
297
 
557 jermar 298
/** Direct handoff of reader/writer lock ownership.
385 jermar 299
 *
300
 * Direct handoff of reader/writer lock ownership
301
 * to waiting readers or a writer.
302
 *
1 jermar 303
 * Must be called with rwl->lock locked.
413 jermar 304
 * Must be called with interrupts_disable()'d.
385 jermar 305
 *
306
 * @param rwl Reader/Writer lock.
307
 * @param readers_only See the description below.
308
 *
1 jermar 309
 * If readers_only is false: (unlock scenario)
310
 * Let the first sleeper on 'exclusive' mutex in, no matter
311
 * whether it is a reader or a writer. If there are more leading
312
 * readers in line, let each of them in.
313
 *
314
 * Otherwise: (timeout scenario)
315
 * Let all leading readers in.
316
 */
317
void let_others_in(rwlock_t *rwl, int readers_only)
318
{
319
    rwlock_type_t type = RWLOCK_NONE;
320
    thread_t *t = NULL;
557 jermar 321
    bool one_more = true;
1 jermar 322
 
323
    spinlock_lock(&rwl->exclusive.sem.wq.lock);
324
 
325
    if (!list_empty(&rwl->exclusive.sem.wq.head))
326
        t = list_get_instance(rwl->exclusive.sem.wq.head.next, thread_t, wq_link);
327
    do {
328
        if (t) {
329
            spinlock_lock(&t->lock);
330
            type = t->rwlock_holder_type;
331
            spinlock_unlock(&t->lock);         
332
        }
333
 
334
        /*
335
         * If readers_only is true, we wake all leading readers
336
         * if and only if rwl is locked by another reader.
337
         * Assumption: readers_only ==> rwl->readers_in
338
         */
339
        if (readers_only && (type != RWLOCK_READER))
340
            break;
341
 
342
 
343
        if (type == RWLOCK_READER) {
344
            /*
345
             * Waking up a reader.
346
             * We are responsible for incrementing rwl->readers_in for it.
347
             */
348
             rwl->readers_in++;
349
        }
350
 
351
        /*
352
         * Only the last iteration through this loop can increment
353
         * rwl->exclusive.sem.wq.missed_wakeup's. All preceeding
354
         * iterations will wake up a thread.
355
         */
356
        /* We call the internal version of waitq_wakeup, which
357
         * relies on the fact that the waitq is already locked.
358
         */
359
        _waitq_wakeup_unsafe(&rwl->exclusive.sem.wq, WAKEUP_FIRST);
360
 
361
        t = NULL;
362
        if (!list_empty(&rwl->exclusive.sem.wq.head)) {
363
            t = list_get_instance(rwl->exclusive.sem.wq.head.next, thread_t, wq_link);
364
            if (t) {
365
                spinlock_lock(&t->lock);
366
                if (t->rwlock_holder_type != RWLOCK_READER)
557 jermar 367
                    one_more = false;
1 jermar 368
                spinlock_unlock(&t->lock); 
369
            }
370
        }
371
    } while ((type == RWLOCK_READER) && t && one_more);
372
 
373
    spinlock_unlock(&rwl->exclusive.sem.wq.lock);
374
}
375
 
385 jermar 376
/** Release spinlock callback
377
 *
378
 * This is a callback function invoked from the scheduler.
379
 * The callback is registered in _rwlock_read_lock_timeout().
380
 *
381
 * @param arg Spinlock.
382
 */
1 jermar 383
void release_spinlock(void *arg)
384
{
385
    spinlock_unlock((spinlock_t *) arg);
386
}
1702 cejka 387
 
1757 jermar 388
/** @}
1702 cejka 389
 */