gnulib/import/m4/pthread_rwlock_rdlock.m4

   1 # pthread_rwlock_rdlock.m4 serial 4
   2 dnl Copyright (C) 2017-2019 Free Software Foundation, Inc.
   3 dnl This file is free software; the Free Software Foundation
   4 dnl gives unlimited permission to copy and/or distribute it,
   5 dnl with or without modifications, as long as this notice is preserved.
   6
   7 dnl From Bruno Haible.
   8 dnl Inspired by
   9 dnl https://github.com/linux-test-project/ltp/blob/master/testcases/open_posix_testsuite/conformance/interfaces/pthread_rwlock_rdlock/2-2.c
  10 dnl by Intel Corporation.
  11
  12 dnl Test whether in a situation where
  13 dnl   - an rwlock is taken by a reader and has a writer waiting,
  14 dnl   - an additional reader requests the lock,
  15 dnl   - the waiting writer and the requesting reader threads have the same
  16 dnl     priority,
  17 dnl the requesting reader thread gets blocked, so that at some point the
  18 dnl waiting writer can acquire the lock.
  19 dnl Without such a guarantee, when there a N readers and each of the readers
  20 dnl spends more than 1/Nth of the time with the lock held, there is a high
  21 dnl probability that the waiting writer will not get the lock in a given finite
  22 dnl time, a phenomenon called "writer starvation".
  23 dnl Without such a guarantee, applications have a hard time avoiding writer
  24 dnl starvation.
  25 dnl
  26 dnl POSIX:2017 makes this requirement only for implementations that support TPS
  27 dnl (Thread Priority Scheduling) and only for the scheduling policies SCHED_FIFO
  28 dnl and SCHED_RR, see
  29 dnl https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_rdlock.html
  30 dnl but this test verifies the guarantee regardless of TPS and regardless of
  31 dnl scheduling policy.
  32 dnl Glibc does not provide this guarantee (and never will on Linux), see
  33 dnl https://sourceware.org/bugzilla/show_bug.cgi?id=13701
  34 dnl https://bugzilla.redhat.com/show_bug.cgi?id=1410052
  35 AC_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
  58 static pthread_rwlock_t lock;
  59 static pthread_t reader1;
  60 static pthread_t writer;
  61 static pthread_t reader2;
  62 static 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.  */
  67 static pthread_mutex_t baton;
  68
  69 static void *
  70 timer_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
  83 static void *
  84 reader2_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
 114 static void *
 115 writer_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
 133 int
 134 main ()
 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 ])