support: Add support_mutex_pi_monotonic

Returns true if Priority Inheritance support CLOCK_MONOTONIC.

support/Makefile

@@ -63,6 +63,7 @@ libsupport-routines = \
   support_format_hostent \
   support_format_netent \
   support_isolate_in_subprocess \
+  support_mutex_pi_monotonic \
   support_path_support_time64 \
   support_process_state \
   support_ptrace \

support/support_mutex_pi_monotonic.c

@@ -0,0 +1,36 @@
+/* Returns whether Priority Inheritance support CLOCK_MONOTONIC.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 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
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <support/xthread.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+bool
+support_mutex_pi_monotonic (void)
+{
+#ifdef __linux__
+# ifndef __NR_futex
+#  define __NR_futex __NR_futex_time64
+# endif
+  int r = syscall (__NR_futex, &(unsigned int){0}, 13 /* FUTEX_LOCK_PI2  */,
+		   0, NULL);
+  return r == 0;
+#else
+  return false;
+#endif
+}

support/xthread.h

@@ -21,6 +21,7 @@
 
 #include <pthread.h>
 #include <sys/cdefs.h>
+#include <stdbool.h>
 
 __BEGIN_DECLS
 
@@ -29,6 +30,9 @@ __BEGIN_DECLS
    exit function, so atexit handlers are executed.  */
 void delayed_exit (int seconds);
 
+/* Returns true if Priority Inheritance support CLOCK_MONOTONIC.  */
+bool support_mutex_pi_monotonic (void);
+
 /* Terminate the process (with exit status 1) if VALUE is not zero.
    In that case, print a failure message to standard output mentioning
    FUNCTION.  The process is terminated with the exit function, so