1  /* Test program for a read-phase / write-phase explicit hand-over.
       2     Copyright (C) 2017-2023 Free Software Foundation, Inc.
       3  
       4     The GNU C Library is free software; you can redistribute it and/or
       5     modify it under the terms of the GNU Lesser General Public License as
       6     published by the Free Software Foundation; either version 2.1 of the
       7     License, or (at your option) any later version.
       8  
       9     The GNU C Library is distributed in the hope that it will be useful,
      10     but WITHOUT ANY WARRANTY; without even the implied warranty of
      11     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
      12     Lesser General Public License for more details.
      13  
      14     You should have received a copy of the GNU Lesser General Public
      15     License along with the GNU C Library; see the file COPYING.LIB.  If
      16     not, see <https://www.gnu.org/licenses/>.  */
      17  
      18  #include <errno.h>
      19  #include <error.h>
      20  #include <pthread.h>
      21  #include <stdio.h>
      22  #include <stdlib.h>
      23  #include <unistd.h>
      24  #include <stdint.h>
      25  #include <time.h>
      26  #include <atomic.h>
      27  #include <support/xthread.h>
      28  
      29  /* We really want to set threads to 2 to reproduce this issue. The goal
      30     is to have one primary writer and a single reader, and to hit the
      31     bug that happens in the interleaving of those two phase transitions.
      32     However, on most hardware, adding a second writer seems to help the
      33     interleaving happen slightly more often, say 20% of the time.  On a
      34     16 core ppc64 machine this fails 100% of the time with an unpatched
      35     glibc.  On a 8 core x86_64 machine this fails ~93% of the time, but
      36     it doesn't fail at all on a 4 core system, so having available
      37     unloaded cores makes a big difference in reproducibility.  On an 8
      38     core qemu/kvm guest the reproducer reliability drops to ~10%.  */
      39  #define THREADS 3
      40  
      41  #define KIND PTHREAD_RWLOCK_PREFER_READER_NP
      42  
      43  static pthread_rwlock_t lock;
      44  static int done = 0;
      45  
      46  static void*
      47  tf (void* arg)
      48  {
      49    while (atomic_load_relaxed (&done) == 0)
      50      {
      51        int rcnt = 0;
      52        int wcnt = 100;
      53        if ((uintptr_t) arg == 0)
      54  	{
      55  	  rcnt = 1;
      56  	  wcnt = 1;
      57  	}
      58  
      59        do
      60  	{
      61  	  if (wcnt)
      62  	    {
      63  	      xpthread_rwlock_wrlock (&lock);
      64  	      xpthread_rwlock_unlock (&lock);
      65  	      wcnt--;
      66  	  }
      67  	  if (rcnt)
      68  	    {
      69  	      xpthread_rwlock_rdlock (&lock);
      70  	      xpthread_rwlock_unlock (&lock);
      71  	      rcnt--;
      72  	  }
      73  	}
      74        while ((atomic_load_relaxed (&done) == 0) && (rcnt + wcnt > 0));
      75  
      76      }
      77      return NULL;
      78  }
      79  
      80  
      81  
      82  static int
      83  do_test (void)
      84  {
      85    pthread_t thr[THREADS];
      86    int n;
      87    pthread_rwlockattr_t attr;
      88  
      89    xpthread_rwlockattr_init (&attr);
      90    xpthread_rwlockattr_setkind_np (&attr, KIND);
      91  
      92    xpthread_rwlock_init (&lock, &attr);
      93  
      94    /* Make standard error the same as standard output.  */
      95    dup2 (1, 2);
      96  
      97    /* Make sure we see all message, even those on stdout.  */
      98    setvbuf (stdout, NULL, _IONBF, 0);
      99  
     100    for (n = 0; n < THREADS; ++n)
     101      thr[n] = xpthread_create (NULL, tf, (void *) (uintptr_t) n);
     102  
     103    struct timespec delay;
     104    delay.tv_sec = 10;
     105    delay.tv_nsec = 0;
     106    nanosleep (&delay, NULL);
     107    atomic_store_relaxed (&done, 1);
     108  
     109    /* Wait for all the threads.  */
     110    for (n = 0; n < THREADS; ++n)
     111      xpthread_join (thr[n]);
     112  
     113    return 0;
     114  }
     115  
     116  #include <support/test-driver.c>