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+# pthread_rwlock_rdlock.m4 serial 4
+dnl Copyright (C) 2017-2022 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+dnl From Bruno Haible.
+dnl Inspired by
+dnl https://github.com/linux-test-project/ltp/blob/master/testcases/open_posix_testsuite/conformance/interfaces/pthread_rwlock_rdlock/2-2.c
+dnl by Intel Corporation.
+
+dnl Test whether in a situation where
+dnl - an rwlock is taken by a reader and has a writer waiting,
+dnl - an additional reader requests the lock,
+dnl - the waiting writer and the requesting reader threads have the same
+dnl priority,
+dnl the requesting reader thread gets blocked, so that at some point the
+dnl waiting writer can acquire the lock.
+dnl Without such a guarantee, when there a N readers and each of the readers
+dnl spends more than 1/Nth of the time with the lock held, there is a high
+dnl probability that the waiting writer will not get the lock in a given finite
+dnl time, a phenomenon called "writer starvation".
+dnl Without such a guarantee, applications have a hard time avoiding writer
+dnl starvation.
+dnl
+dnl POSIX:2017 makes this requirement only for implementations that support TPS
+dnl (Thread Priority Scheduling) and only for the scheduling policies SCHED_FIFO
+dnl and SCHED_RR, see
+dnl https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_rdlock.html
+dnl but this test verifies the guarantee regardless of TPS and regardless of
+dnl scheduling policy.
+dnl Glibc does not provide this guarantee (and never will on Linux), see
+dnl https://sourceware.org/bugzilla/show_bug.cgi?id=13701
+dnl https://bugzilla.redhat.com/show_bug.cgi?id=1410052
+AC_DEFUN([gl_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER],
+[
+ AC_REQUIRE([gl_THREADLIB_EARLY])
+ AC_REQUIRE([AC_CANONICAL_HOST]) dnl for cross-compiles
+ AC_CACHE_CHECK([whether pthread_rwlock_rdlock prefers a writer to a reader],
+ [gl_cv_pthread_rwlock_rdlock_prefer_writer],
+ [save_LIBS="$LIBS"
+ LIBS="$LIBS $LIBMULTITHREAD"
+ AC_RUN_IFELSE(
+ [AC_LANG_SOURCE([[
+#include <errno.h>
+#include <pthread.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#define SUCCEED() exit (0)
+#define FAILURE() exit (1)
+#define UNEXPECTED(n) (exit (10 + (n)))
+
+/* The main thread creates the waiting writer and the requesting reader threads
+ in the default way; this guarantees that they have the same priority.
+ We can reuse the main thread as first reader thread. */
+
+static pthread_rwlock_t lock;
+static pthread_t reader1;
+static pthread_t writer;
+static pthread_t reader2;
+static pthread_t timer;
+/* Used to pass control from writer to reader2 and from reader2 to timer,
+ as in a relay race.
+ Passing control from one running thread to another running thread
+ is most likely faster than to create the second thread. */
+static pthread_mutex_t baton;
+
+static void *
+timer_func (void *ignored)
+{
+ /* Step 13 (can be before or after step 12):
+ The timer thread takes the baton, then waits a moment to make sure
+ it can tell whether the second reader thread is blocked at step 12. */
+ if (pthread_mutex_lock (&baton))
+ UNEXPECTED (13);
+ usleep (100000);
+ /* By the time we get here, it's clear that the second reader thread is
+ blocked at step 12. This is the desired behaviour. */
+ SUCCEED ();
+}
+
+static void *
+reader2_func (void *ignored)
+{
+ int err;
+
+ /* Step 8 (can be before or after step 7):
+ The second reader thread takes the baton, then waits a moment to make sure
+ the writer thread has reached step 7. */
+ if (pthread_mutex_lock (&baton))
+ UNEXPECTED (8);
+ usleep (100000);
+ /* Step 9: The second reader thread requests the lock. */
+ err = pthread_rwlock_tryrdlock (&lock);
+ if (err == 0)
+ FAILURE ();
+ else if (err != EBUSY)
+ UNEXPECTED (9);
+ /* Step 10: Launch a timer, to test whether the next call blocks. */
+ if (pthread_create (&timer, NULL, timer_func, NULL))
+ UNEXPECTED (10);
+ /* Step 11: Release the baton. */
+ if (pthread_mutex_unlock (&baton))
+ UNEXPECTED (11);
+ /* Step 12: The second reader thread requests the lock. */
+ err = pthread_rwlock_rdlock (&lock);
+ if (err == 0)
+ FAILURE ();
+ else
+ UNEXPECTED (12);
+}
+
+static void *
+writer_func (void *ignored)
+{
+ /* Step 4: Take the baton, so that the second reader thread does not go ahead
+ too early. */
+ if (pthread_mutex_lock (&baton))
+ UNEXPECTED (4);
+ /* Step 5: Create the second reader thread. */
+ if (pthread_create (&reader2, NULL, reader2_func, NULL))
+ UNEXPECTED (5);
+ /* Step 6: Release the baton. */
+ if (pthread_mutex_unlock (&baton))
+ UNEXPECTED (6);
+ /* Step 7: The writer thread requests the lock. */
+ if (pthread_rwlock_wrlock (&lock))
+ UNEXPECTED (7);
+ return NULL;
+}
+
+int
+main ()
+{
+ reader1 = pthread_self ();
+
+ /* Step 1: The main thread initializes the lock and the baton. */
+ if (pthread_rwlock_init (&lock, NULL))
+ UNEXPECTED (1);
+ if (pthread_mutex_init (&baton, NULL))
+ UNEXPECTED (1);
+ /* Step 2: The main thread acquires the lock as a reader. */
+ if (pthread_rwlock_rdlock (&lock))
+ UNEXPECTED (2);
+ /* Step 3: Create the writer thread. */
+ if (pthread_create (&writer, NULL, writer_func, NULL))
+ UNEXPECTED (3);
+ /* Job done. Go to sleep. */
+ for (;;)
+ {
+ sleep (1);
+ }
+}
+]])],
+ [gl_cv_pthread_rwlock_rdlock_prefer_writer=yes],
+ [gl_cv_pthread_rwlock_rdlock_prefer_writer=no],
+ [case "$host_os" in
+ # Guess no on glibc systems.
+ *-gnu* | gnu*) gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing no" ;;
+ # Guess no on musl systems.
+ *-musl*) gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing no" ;;
+ # Guess no on bionic systems.
+ *-android*) gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing no" ;;
+ # Guess yes on native Windows with the mingw-w64 winpthreads library.
+ # Guess no on native Windows with the gnulib windows-rwlock module.
+ mingw*) if test "$gl_use_threads" = yes || test "$gl_use_threads" = posix; then
+ gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing yes"
+ else
+ gl_cv_pthread_rwlock_rdlock_prefer_writer="guessing no"
+ fi
+ ;;
+ # If we don't know, obey --enable-cross-guesses.
+ *) gl_cv_pthread_rwlock_rdlock_prefer_writer="$gl_cross_guess_normal" ;;
+ esac
+ ])
+ LIBS="$save_LIBS"
+ ])
+ case "$gl_cv_pthread_rwlock_rdlock_prefer_writer" in
+ *yes)
+ AC_DEFINE([HAVE_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER], [1],
+ [Define if the 'pthread_rwlock_rdlock' function prefers a writer to a reader.])
+ ;;
+ esac
+])