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1# pthread_rwlock_rdlock.m4 serial 4
2dnl Copyright (C) 2017-2023 Free Software Foundation, Inc.
3dnl This file is free software; the Free Software Foundation
4dnl gives unlimited permission to copy and/or distribute it,
5dnl with or without modifications, as long as this notice is preserved.
6
7dnl From Bruno Haible.
8dnl Inspired by
9dnl https://github.com/linux-test-project/ltp/blob/master/testcases/open_posix_testsuite/conformance/interfaces/pthread_rwlock_rdlock/2-2.c
10dnl by Intel Corporation.
11
12dnl Test whether in a situation where
13dnl - an rwlock is taken by a reader and has a writer waiting,
14dnl - an additional reader requests the lock,
15dnl - the waiting writer and the requesting reader threads have the same
16dnl priority,
17dnl the requesting reader thread gets blocked, so that at some point the
18dnl waiting writer can acquire the lock.
19dnl Without such a guarantee, when there a N readers and each of the readers
20dnl spends more than 1/Nth of the time with the lock held, there is a high
21dnl probability that the waiting writer will not get the lock in a given finite
22dnl time, a phenomenon called "writer starvation".
23dnl Without such a guarantee, applications have a hard time avoiding writer
24dnl starvation.
25dnl
26dnl POSIX:2017 makes this requirement only for implementations that support TPS
27dnl (Thread Priority Scheduling) and only for the scheduling policies SCHED_FIFO
28dnl and SCHED_RR, see
29dnl https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_rdlock.html
30dnl but this test verifies the guarantee regardless of TPS and regardless of
31dnl scheduling policy.
32dnl Glibc does not provide this guarantee (and never will on Linux), see
33dnl https://sourceware.org/bugzilla/show_bug.cgi?id=13701
34dnl https://bugzilla.redhat.com/show_bug.cgi?id=1410052
35AC_DEFUN([gl_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER],
36[
37 AC_REQUIRE([gl_THREADLIB_EARLY])
38 AC_REQUIRE([AC_CANONICAL_HOST]) dnl for cross-compiles
39 AC_CACHE_CHECK([whether pthread_rwlock_rdlock prefers a writer to a reader],
40 [gl_cv_pthread_rwlock_rdlock_prefer_writer],
41 [save_LIBS="$LIBS"
42 LIBS="$LIBS $LIBMULTITHREAD"
43 AC_RUN_IFELSE(
44 [AC_LANG_SOURCE([[
45#include <errno.h>
46#include <pthread.h>
47#include <stdlib.h>
48#include <unistd.h>
49
50#define SUCCEED() exit (0)
51#define FAILURE() exit (1)
52#define UNEXPECTED(n) (exit (10 + (n)))
53
54/* The main thread creates the waiting writer and the requesting reader threads
55 in the default way; this guarantees that they have the same priority.
56 We can reuse the main thread as first reader thread. */
57
58static pthread_rwlock_t lock;
59static pthread_t reader1;
60static pthread_t writer;
61static pthread_t reader2;
62static pthread_t timer;
63/* Used to pass control from writer to reader2 and from reader2 to timer,
64 as in a relay race.
65 Passing control from one running thread to another running thread
66 is most likely faster than to create the second thread. */
67static pthread_mutex_t baton;
68
69static void *
70timer_func (void *ignored)
71{
72 /* Step 13 (can be before or after step 12):
73 The timer thread takes the baton, then waits a moment to make sure
74 it can tell whether the second reader thread is blocked at step 12. */
75 if (pthread_mutex_lock (&baton))
76 UNEXPECTED (13);
77 usleep (100000);
78 /* By the time we get here, it's clear that the second reader thread is
79 blocked at step 12. This is the desired behaviour. */
80 SUCCEED ();
81}
82
83static void *
84reader2_func (void *ignored)
85{
86 int err;
87
88 /* Step 8 (can be before or after step 7):
89 The second reader thread takes the baton, then waits a moment to make sure
90 the writer thread has reached step 7. */
91 if (pthread_mutex_lock (&baton))
92 UNEXPECTED (8);
93 usleep (100000);
94 /* Step 9: The second reader thread requests the lock. */
95 err = pthread_rwlock_tryrdlock (&lock);
96 if (err == 0)
97 FAILURE ();
98 else if (err != EBUSY)
99 UNEXPECTED (9);
100 /* Step 10: Launch a timer, to test whether the next call blocks. */
101 if (pthread_create (&timer, NULL, timer_func, NULL))
102 UNEXPECTED (10);
103 /* Step 11: Release the baton. */
104 if (pthread_mutex_unlock (&baton))
105 UNEXPECTED (11);
106 /* Step 12: The second reader thread requests the lock. */
107 err = pthread_rwlock_rdlock (&lock);
108 if (err == 0)
109 FAILURE ();
110 else
111 UNEXPECTED (12);
112}
113
114static void *
115writer_func (void *ignored)
116{
117 /* Step 4: Take the baton, so that the second reader thread does not go ahead
118 too early. */
119 if (pthread_mutex_lock (&baton))
120 UNEXPECTED (4);
121 /* Step 5: Create the second reader thread. */
122 if (pthread_create (&reader2, NULL, reader2_func, NULL))
123 UNEXPECTED (5);
124 /* Step 6: Release the baton. */
125 if (pthread_mutex_unlock (&baton))
126 UNEXPECTED (6);
127 /* Step 7: The writer thread requests the lock. */
128 if (pthread_rwlock_wrlock (&lock))
129 UNEXPECTED (7);
130 return NULL;
131}
132
133int
134main ()
135{
136 reader1 = pthread_self ();
137
138 /* Step 1: The main thread initializes the lock and the baton. */
139 if (pthread_rwlock_init (&lock, NULL))
140 UNEXPECTED (1);
141 if (pthread_mutex_init (&baton, NULL))
142 UNEXPECTED (1);
143 /* Step 2: The main thread acquires the lock as a reader. */
144 if (pthread_rwlock_rdlock (&lock))
145 UNEXPECTED (2);
146 /* Step 3: Create the writer thread. */
147 if (pthread_create (&writer, NULL, writer_func, NULL))
148 UNEXPECTED (3);
149 /* Job done. Go to sleep. */
150 for (;;)
151 {
152 sleep (1);
153 }
154}
155]])],
156 [gl_cv_pthread_rwlock_rdlock_prefer_writer=yes],
157 [gl_cv_pthread_rwlock_rdlock_prefer_writer=no],
158 [case "$host_os" in
159 # Guess no on glibc systems.
160 *-gnu* | gnu*) gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing no" ;;
161 # Guess no on musl systems.
162 *-musl*) gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing no" ;;
163 # Guess no on bionic systems.
164 *-android*) gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing no" ;;
165 # Guess yes on native Windows with the mingw-w64 winpthreads library.
166 # Guess no on native Windows with the gnulib windows-rwlock module.
167 mingw*) if test "$gl_use_threads" = yes || test "$gl_use_threads" = posix; then
168 gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing yes"
169 else
170 gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing no"
171 fi
172 ;;
173 # If we don't know, obey --enable-cross-guesses.
174 *) gl_cv_pthread_rwlock_rdlock_prefer_writer="$gl_cross_guess_normal" ;;
175 esac
176 ])
177 LIBS="$save_LIBS"
178 ])
179 case "$gl_cv_pthread_rwlock_rdlock_prefer_writer" in
180 *yes)
181 AC_DEFINE([HAVE_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER], [1],
182 [Define if the 'pthread_rwlock_rdlock' function prefers a writer to a reader.])
183 ;;
184 esac
185])