summary refs log tree commit diff
path: root/linuxthreads/rwlock.c
blob: e4a4c81f8cad1a458c0cc4339b36708a60822e76 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
/* Read-write lock implementation.
   Copyright (C) 1998, 2000 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Xavier Leroy <Xavier.Leroy@inria.fr>
   and Ulrich Drepper <drepper@cygnus.com>, 1998.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Library General Public License as
   published by the Free Software Foundation; either version 2 of the
   License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Library General Public License for more details.

   You should have received a copy of the GNU Library General Public
   License along with the GNU C Library; see the file COPYING.LIB.  If not,
   write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
   Boston, MA 02111-1307, USA.  */

#include <errno.h>
#include <pthread.h>
#include <stdlib.h>
#include "internals.h"
#include "queue.h"
#include "spinlock.h"
#include "restart.h"

/*
 * Check whether the calling thread already owns one or more read locks on the
 * specified lock. If so, return a pointer to the read lock info structure
 * corresponding to that lock.
 */

static pthread_readlock_info *
rwlock_is_in_list(pthread_descr self, pthread_rwlock_t *rwlock)
{
  pthread_readlock_info *info;

  for (info = self->p_readlock_list; info != NULL; info = info->pr_next)
    {
      if (info->pr_lock == rwlock)
	return info;
    }

  return NULL;
}

/*
 * Add a new lock to the thread's list of locks for which it has a read lock.
 * A new info node must be allocated for this, which is taken from the thread's
 * free list, or by calling malloc. If malloc fails, a null pointer is
 * returned. Otherwise the lock info structure is initialized and pushed
 * onto the thread's list.
 */

static pthread_readlock_info *
rwlock_add_to_list(pthread_descr self, pthread_rwlock_t *rwlock)
{
  pthread_readlock_info *info = self->p_readlock_free;

  if (info != NULL)
    self->p_readlock_free = info->pr_next;
  else
    info = malloc(sizeof *info);

  if (info == NULL)
    return NULL;

  info->pr_lock_count = 1;
  info->pr_lock = rwlock;
  info->pr_next = self->p_readlock_list;
  self->p_readlock_list = info;

  return info;
}

/*
 * If the thread owns a read lock over the given pthread_rwlock_t,
 * and this read lock is tracked in the thread's lock list,
 * this function returns a pointer to the info node in that list.
 * It also decrements the lock count within that node, and if
 * it reaches zero, it removes the node from the list.
 * If nothing is found, it returns a null pointer.
 */

static pthread_readlock_info *
rwlock_remove_from_list(pthread_descr self, pthread_rwlock_t *rwlock)
{
  pthread_readlock_info **pinfo;

  for (pinfo = &self->p_readlock_list; *pinfo != NULL; pinfo = &(*pinfo)->pr_next)
    {
      if ((*pinfo)->pr_lock == rwlock)
	{
	  pthread_readlock_info *info = *pinfo;
	  if (--info->pr_lock_count == 0)
	    *pinfo = info->pr_next;
	  return info;
	}
    }

  return NULL;
}

/*
 * This function checks whether the conditions are right to place a read lock.
 * It returns 1 if so, otherwise zero. The rwlock's internal lock must be
 * locked upon entry.
 */

static int
rwlock_can_rdlock(pthread_rwlock_t *rwlock, int have_lock_already)
{
  /* Can't readlock; it is write locked. */
  if (rwlock->__rw_writer != NULL)
    return 0;

  /* Lock prefers readers; get it. */
  if (rwlock->__rw_kind == PTHREAD_RWLOCK_PREFER_READER_NP)
    return 1;

  /* Lock prefers writers, but none are waiting. */
  if (queue_is_empty(&rwlock->__rw_write_waiting))
    return 1;

  /* Writers are waiting, but this thread already has a read lock */
  if (have_lock_already)
    return 1;

  /* Writers are waiting, and this is a new lock */
  return 0;
}

/*
 * This function helps support brain-damaged recursive read locking
 * semantics required by Unix 98, while maintaining write priority.
 * This basically determines whether this thread already holds a read lock
 * already. It returns 1 if so, otherwise it returns 0.
 *
 * If the thread has any ``untracked read locks'' then it just assumes
 * that this lock is among them, just to be safe, and returns 1.
 *
 * Also, if it finds the thread's lock in the list, it sets the pointer
 * referenced by pexisting to refer to the list entry.
 *
 * If the thread has no untracked locks, and the lock is not found
 * in its list, then it is added to the list. If this fails,
 * then *pout_of_mem is set to 1.
 */

static int
rwlock_have_already(pthread_descr *pself, pthread_rwlock_t *rwlock,
    pthread_readlock_info **pexisting, int *pout_of_mem)
{
  pthread_readlock_info *existing = NULL;
  int out_of_mem = 0, have_lock_already = 0;
  pthread_descr self = *pself;

  if (rwlock->__rw_kind == PTHREAD_RWLOCK_PREFER_WRITER_NP)
    {
      if (!self)
	self = thread_self();

      existing = rwlock_is_in_list(self, rwlock);

      if (existing != NULL || self->p_untracked_readlock_count > 0)
	have_lock_already = 1;
      else
	{
	  existing = rwlock_add_to_list(self, rwlock);
	  if (existing == NULL)
	    out_of_mem = 1;
	}
    }

  *pout_of_mem = out_of_mem;
  *pexisting = existing;
  *pself = self;

  return have_lock_already;
}

int
__pthread_rwlock_init (pthread_rwlock_t *rwlock,
		     const pthread_rwlockattr_t *attr)
{
  __pthread_init_lock(&rwlock->__rw_lock);
  rwlock->__rw_readers = 0;
  rwlock->__rw_writer = NULL;
  rwlock->__rw_read_waiting = NULL;
  rwlock->__rw_write_waiting = NULL;

  if (attr == NULL)
    {
      rwlock->__rw_kind = PTHREAD_RWLOCK_DEFAULT_NP;
      rwlock->__rw_pshared = PTHREAD_PROCESS_PRIVATE;
    }
  else
    {
      rwlock->__rw_kind = attr->__lockkind;
      rwlock->__rw_pshared = attr->__pshared;
    }

  return 0;
}
strong_alias (__pthread_rwlock_init, pthread_rwlock_init)


int
__pthread_rwlock_destroy (pthread_rwlock_t *rwlock)
{
  int readers;
  _pthread_descr writer;

  __pthread_lock (&rwlock->__rw_lock, NULL);
  readers = rwlock->__rw_readers;
  writer = rwlock->__rw_writer;
  __pthread_spin_unlock (&rwlock->__rw_lock);

  if (readers > 0 || writer != NULL)
    return EBUSY;

  return 0;
}
strong_alias (__pthread_rwlock_destroy, pthread_rwlock_destroy)

int
__pthread_rwlock_rdlock (pthread_rwlock_t *rwlock)
{
  pthread_descr self = NULL;
  pthread_readlock_info *existing;
  int out_of_mem, have_lock_already;

  have_lock_already = rwlock_have_already(&self, rwlock,
					  &existing, &out_of_mem);

  for (;;)
    {
      if (self == NULL)
	self = thread_self ();

      __pthread_lock (&rwlock->__rw_lock, self);

      if (rwlock_can_rdlock(rwlock, have_lock_already))
	break;

      enqueue (&rwlock->__rw_read_waiting, self);
      __pthread_spin_unlock (&rwlock->__rw_lock);
      suspend (self); /* This is not a cancellation point */
    }

  ++rwlock->__rw_readers;
  __pthread_spin_unlock (&rwlock->__rw_lock);

  if (have_lock_already || out_of_mem)
    {
      if (existing != NULL)
	existing->pr_lock_count++;
      else
	self->p_untracked_readlock_count++;
    }

  return 0;
}
strong_alias (__pthread_rwlock_rdlock, pthread_rwlock_rdlock)

int
__pthread_rwlock_tryrdlock (pthread_rwlock_t *rwlock)
{
  pthread_descr self = thread_self();
  pthread_readlock_info *existing;
  int out_of_mem, have_lock_already;
  int retval = EBUSY;

  have_lock_already = rwlock_have_already(&self, rwlock,
      &existing, &out_of_mem);

  __pthread_lock (&rwlock->__rw_lock, self);

  /* 0 is passed to here instead of have_lock_already.
     This is to meet Single Unix Spec requirements:
     if writers are waiting, pthread_rwlock_tryrdlock
     does not acquire a read lock, even if the caller has
     one or more read locks already. */

  if (rwlock_can_rdlock(rwlock, 0))
    {
      ++rwlock->__rw_readers;
      retval = 0;
    }

  __pthread_spin_unlock (&rwlock->__rw_lock);

  if (retval == 0)
    {
      if (have_lock_already || out_of_mem)
	{
	  if (existing != NULL)
	    existing->pr_lock_count++;
	  else
	    self->p_untracked_readlock_count++;
	}
    }

  return retval;
}
strong_alias (__pthread_rwlock_tryrdlock, pthread_rwlock_tryrdlock)


int
__pthread_rwlock_wrlock (pthread_rwlock_t *rwlock)
{
  pthread_descr self = thread_self ();

  while(1)
    {
      __pthread_lock (&rwlock->__rw_lock, self);
      if (rwlock->__rw_readers == 0 && rwlock->__rw_writer == NULL)
	{
	  rwlock->__rw_writer = self;
	  __pthread_spin_unlock (&rwlock->__rw_lock);
	  return 0;
	}

      /* Suspend ourselves, then try again */
      enqueue (&rwlock->__rw_write_waiting, self);
      __pthread_spin_unlock (&rwlock->__rw_lock);
      suspend (self); /* This is not a cancellation point */
    }
}
strong_alias (__pthread_rwlock_wrlock, pthread_rwlock_wrlock)


int
__pthread_rwlock_trywrlock (pthread_rwlock_t *rwlock)
{
  int result = EBUSY;

  __pthread_lock (&rwlock->__rw_lock, NULL);
  if (rwlock->__rw_readers == 0 && rwlock->__rw_writer == NULL)
    {
      rwlock->__rw_writer = thread_self ();
      result = 0;
    }
  __pthread_spin_unlock (&rwlock->__rw_lock);

  return result;
}
strong_alias (__pthread_rwlock_trywrlock, pthread_rwlock_trywrlock)


int
__pthread_rwlock_unlock (pthread_rwlock_t *rwlock)
{
  pthread_descr torestart;
  pthread_descr th;

  __pthread_lock (&rwlock->__rw_lock, NULL);
  if (rwlock->__rw_writer != NULL)
    {
      /* Unlocking a write lock.  */
      if (rwlock->__rw_writer != thread_self ())
	{
	  __pthread_spin_unlock (&rwlock->__rw_lock);
	  return EPERM;
	}
      rwlock->__rw_writer = NULL;

      if ((rwlock->__rw_kind == PTHREAD_RWLOCK_PREFER_READER_NP
	   && !queue_is_empty(&rwlock->__rw_read_waiting))
	  || (th = dequeue(&rwlock->__rw_write_waiting)) == NULL)
	{
	  /* Restart all waiting readers.  */
	  torestart = rwlock->__rw_read_waiting;
	  rwlock->__rw_read_waiting = NULL;
	  __pthread_spin_unlock (&rwlock->__rw_lock);
	  while ((th = dequeue (&torestart)) != NULL)
	    restart (th);
	}
      else
	{
	  /* Restart one waiting writer.  */
	  __pthread_spin_unlock (&rwlock->__rw_lock);
	  restart (th);
	}
    }
  else
    {
      /* Unlocking a read lock.  */
      if (rwlock->__rw_readers == 0)
	{
	  __pthread_spin_unlock (&rwlock->__rw_lock);
	  return EPERM;
	}

      --rwlock->__rw_readers;
      if (rwlock->__rw_readers == 0)
	/* Restart one waiting writer, if any.  */
	th = dequeue (&rwlock->__rw_write_waiting);
      else
	th = NULL;

      __pthread_spin_unlock (&rwlock->__rw_lock);
      if (th != NULL)
	restart (th);

      /* Recursive lock fixup */

      if (rwlock->__rw_kind == PTHREAD_RWLOCK_PREFER_WRITER_NP)
	{
	  pthread_descr self = thread_self();
	  pthread_readlock_info *victim = rwlock_remove_from_list(self, rwlock);

	  if (victim != NULL)
	    {
	      if (victim->pr_lock_count == 0)
		{
		  victim->pr_next = self->p_readlock_free;
		  self->p_readlock_free = victim;
		}
	    }
	  else
	    {
	      if (self->p_untracked_readlock_count > 0)
		self->p_untracked_readlock_count--;
	    }
	}
    }

  return 0;
}
strong_alias (__pthread_rwlock_unlock, pthread_rwlock_unlock)



int
pthread_rwlockattr_init (pthread_rwlockattr_t *attr)
{
  attr->__lockkind = 0;
  attr->__pshared = 0;

  return 0;
}


int
__pthread_rwlockattr_destroy (pthread_rwlockattr_t *attr)
{
  return 0;
}
strong_alias (__pthread_rwlockattr_destroy, pthread_rwlockattr_destroy)


int
pthread_rwlockattr_getpshared (const pthread_rwlockattr_t *attr, int *pshared)
{
  *pshared = attr->__pshared;
  return 0;
}


int
pthread_rwlockattr_setpshared (pthread_rwlockattr_t *attr, int pshared)
{
  if (pshared != PTHREAD_PROCESS_PRIVATE && pshared != PTHREAD_PROCESS_SHARED)
    return EINVAL;

  attr->__pshared = pshared;

  return 0;
}


int
pthread_rwlockattr_getkind_np (const pthread_rwlockattr_t *attr, int *pref)
{
  *pref = attr->__lockkind;
  return 0;
}


int
pthread_rwlockattr_setkind_np (pthread_rwlockattr_t *attr, int pref)
{
  if (pref != PTHREAD_RWLOCK_PREFER_READER_NP
      && pref != PTHREAD_RWLOCK_PREFER_WRITER_NP
      && pref != PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP
      && pref != PTHREAD_RWLOCK_DEFAULT_NP)
    return EINVAL;

  attr->__lockkind = pref;

  return 0;
}