1 /* Copyright (C) 2003, 2004, 2007 Free Software Foundation, Inc.
2 This file is part of the GNU C Library.
3 Contributed by Martin Schwidefsky <schwidefsky@de.ibm.com>, 2003.
4
5 The GNU C Library is free software; you can redistribute it and/or
6 modify it under the terms of the GNU Lesser General Public
7 License as published by the Free Software Foundation; either
8 version 2.1 of the License, or (at your option) any later version.
9
10 The GNU C Library is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 Lesser General Public License for more details.
14
15 You should have received a copy of the GNU Lesser General Public
16 License along with the GNU C Library; if not, see
17 <http://www.gnu.org/licenses/>. */
18
19 #include <errno.h>
20 #include <sysdep.h>
21 #include <lowlevellock.h>
22 #include <pthread.h>
23 #include <pthreadP.h>
24
25
26 /* Acquire read lock for RWLOCK. */
27 int
28 attribute_protected
__pthread_rwlock_rdlock(pthread_rwlock_t * rwlock)29 __pthread_rwlock_rdlock (
30 pthread_rwlock_t *rwlock)
31 {
32 int result = 0;
33
34 /* Make sure we are along. */
35 lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
36
37 while (1)
38 {
39 /* Get the rwlock if there is no writer... */
40 if (rwlock->__data.__writer == 0
41 /* ...and if either no writer is waiting or we prefer readers. */
42 && (!rwlock->__data.__nr_writers_queued
43 || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
44 {
45 /* Increment the reader counter. Avoid overflow. */
46 if (__builtin_expect (++rwlock->__data.__nr_readers == 0, 0))
47 {
48 /* Overflow on number of readers. */
49 --rwlock->__data.__nr_readers;
50 result = EAGAIN;
51 }
52
53 break;
54 }
55
56 /* Make sure we are not holding the rwlock as a writer. This is
57 a deadlock situation we recognize and report. */
58 if (__builtin_expect (rwlock->__data.__writer
59 == THREAD_GETMEM (THREAD_SELF, tid), 0))
60 {
61 result = EDEADLK;
62 break;
63 }
64
65 /* Remember that we are a reader. */
66 if (__builtin_expect (++rwlock->__data.__nr_readers_queued == 0, 0))
67 {
68 /* Overflow on number of queued readers. */
69 --rwlock->__data.__nr_readers_queued;
70 result = EAGAIN;
71 break;
72 }
73
74 int waitval = rwlock->__data.__readers_wakeup;
75
76 /* Free the lock. */
77 lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
78
79 /* Wait for the writer to finish. */
80 lll_futex_wait (&rwlock->__data.__readers_wakeup, waitval,
81 rwlock->__data.__shared);
82
83 /* Get the lock. */
84 lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
85
86 --rwlock->__data.__nr_readers_queued;
87 }
88
89 /* We are done, free the lock. */
90 lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
91
92 return result;
93 }
94
95 weak_alias (__pthread_rwlock_rdlock, pthread_rwlock_rdlock)
96 strong_alias (__pthread_rwlock_rdlock, __pthread_rwlock_rdlock_internal)
97