1/* Acquire a rwlock for reading. Generic version.
2 Copyright (C) 2002-2024 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
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 <https://www.gnu.org/licenses/>. */
18
19#include <pthread.h>
20#include <assert.h>
21#include <time.h>
22
23#include <pt-internal.h>
24
25/* Acquire the rwlock *RWLOCK for reading blocking until *ABSTIME if
26 it is already held. As a GNU extension, if TIMESPEC is NULL then
27 wait forever. */
28int
29__pthread_rwlock_timedrdlock_internal (struct __pthread_rwlock *rwlock,
30 clockid_t clockid,
31 const struct timespec *abstime)
32{
33 error_t err;
34 int drain;
35 struct __pthread *self;
36
37 __pthread_spin_wait (&rwlock->__lock);
38 if (__pthread_spin_trylock (&rwlock->__held) == 0)
39 /* Successfully acquired the lock. */
40 {
41 assert (rwlock->__readerqueue == 0);
42 assert (rwlock->__writerqueue == 0);
43 assert (rwlock->__readers == 0);
44
45 rwlock->__readers = 1;
46 __pthread_spin_unlock (&rwlock->__lock);
47 return 0;
48 }
49 else
50 /* Lock is held, but is held by a reader? */
51 if (rwlock->__readers > 0)
52 /* Just add ourself to number of readers. */
53 {
54 assert (rwlock->__readerqueue == 0);
55 rwlock->__readers++;
56 __pthread_spin_unlock (&rwlock->__lock);
57 return 0;
58 }
59
60 /* The lock is busy. */
61
62 /* Better be blocked by a writer. */
63 assert (rwlock->__readers == 0);
64
65 if (abstime != NULL && ! valid_nanoseconds (ns: abstime->tv_nsec))
66 {
67 __pthread_spin_unlock (&rwlock->__lock);
68 return EINVAL;
69 }
70
71 self = _pthread_self ();
72
73 /* Add ourself to the queue. */
74 __pthread_enqueue (&rwlock->__readerqueue, self);
75 __pthread_spin_unlock (&rwlock->__lock);
76
77 /* Block the thread. */
78 if (abstime != NULL)
79 err = __pthread_timedblock (self, abstime, clockid);
80 else
81 {
82 err = 0;
83 __pthread_block (self);
84 }
85
86 __pthread_spin_wait (&rwlock->__lock);
87 if (self->prevp == NULL)
88 /* Another thread removed us from the queue, which means a wakeup message
89 has been sent. It was either consumed while we were blocking, or
90 queued after we timed out and before we acquired the rwlock lock, in
91 which case the message queue must be drained. */
92 drain = err ? 1 : 0;
93 else
94 {
95 /* We're still in the queue. No one attempted to wake us up, i.e. we
96 timed out. */
97 __pthread_dequeue (self);
98 drain = 0;
99 }
100 __pthread_spin_unlock (&rwlock->__lock);
101
102 if (drain)
103 __pthread_block (self);
104
105 if (err)
106 {
107 assert (err == ETIMEDOUT);
108 return err;
109 }
110
111 /* The reader count has already been increment by whoever woke us
112 up. */
113
114 assert (rwlock->__readers > 0);
115
116 return 0;
117}
118
119int
120__pthread_rwlock_timedrdlock (struct __pthread_rwlock *rwlock,
121 const struct timespec *abstime)
122{
123 return __pthread_rwlock_timedrdlock_internal (rwlock, CLOCK_REALTIME, abstime);
124}
125weak_alias (__pthread_rwlock_timedrdlock, pthread_rwlock_timedrdlock)
126
127int
128__pthread_rwlock_clockrdlock (struct __pthread_rwlock *rwlock,
129 clockid_t clockid,
130 const struct timespec *abstime)
131{
132 return __pthread_rwlock_timedrdlock_internal (rwlock, clockid, abstime);
133}
134weak_alias (__pthread_rwlock_clockrdlock, pthread_rwlock_clockrdlock)
135

source code of glibc/sysdeps/htl/pt-rwlock-timedrdlock.c