/* Bug 23844: Test for pthread_rwlock_trywrlock stalls.

   Copyright (C) 2019 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

   <http://www.gnu.org/licenses/>.  */

/* For a full analysis see comments in tst-rwlock-tryrdlock-stall.c.

   Summary for the pthread_rwlock_trywrlock() stall:

   The stall is caused by pthread_rwlock_trywrlock setting

   __wrphase_futex futex to 1 and loosing the

   PTHREAD_RWLOCK_FUTEX_USED bit.

   The fix for bug 23844 ensures that waiters on __wrphase_futex are

   correctly woken.  Before the fix the test stalls as readers can

   wait forever on  __wrphase_futex.  */

#include <stdio.h>

#include <stdlib.h>

#include <unistd.h>

#include <pthread.h>

#include <support/xthread.h>

#include <errno.h>

/* We need only one lock to reproduce the issue. We will need multiple

   threads to get the exact case where we have a read, try, and unlock

   all interleaving to produce the case where the readers are waiting

   and the try clears the PTHREAD_RWLOCK_FUTEX_USED bit and a

   subsequent unlock fails to wake them.  */

pthread_rwlock_t onelock;

/* The number of threads is arbitrary but empirically chosen to have

   enough threads that we see the condition where waiting readers are

   not woken by a successful unlock.  */

#define NTHREADS 32

_Atomic int do_exit;

void *

run_loop (void *arg)

{

  int i = 0, ret;

  while (!do_exit)

    {

      /* Arbitrarily choose if we are the writer or reader.  Choose a

	 high enough ratio of readers to writers to make it likely

	 that readers block (and eventually are susceptable to

	 stalling).

         If we are a writer, take the write lock, and then unlock.

	 If we are a reader, try the lock, then lock, then unlock.  */

      if ((i % 8) != 0)

	{

	  if ((ret = pthread_rwlock_trywrlock (&onelock)) != 0)

	    {

	      if (ret == EBUSY)

		xpthread_rwlock_wrlock (&onelock);

	      else

		exit (EXIT_FAILURE);

	    }

	}

      else

	xpthread_rwlock_rdlock (&onelock);

      /* Thread does some work and then unlocks.  */

      xpthread_rwlock_unlock (&onelock);

      i++;

    }

  return NULL;

}

int

do_test (void)

{

  int i;

  pthread_t tids[NTHREADS];

  xpthread_rwlock_init (&onelock, NULL);

  for (i = 0; i < NTHREADS; i++)

    tids[i] = xpthread_create (NULL, run_loop, NULL);

  /* Run for some amount of time.  The pthread_rwlock_tryrwlock stall

     is very easy to trigger and happens in seconds under the test

     conditions.  */

  sleep (10);

  /* Then exit.  */

  printf ("INFO: Exiting...\n");

  do_exit = 1;

  /* If any readers stalled then we will timeout waiting for them.  */

  for (i = 0; i < NTHREADS; i++)

    xpthread_join (tids[i]);

  printf ("INFO: Done.\n");

  xpthread_rwlock_destroy (&onelock);

  printf ("PASS: No pthread_rwlock_tryrwlock stalls detected.\n");

  return 0;

}

#include <support/test-driver.c>