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/* Read-write lock implementation.

   Copyright (C) 1998 Free Software Foundation, Inc.

   This file is part of the GNU C Library.

   Contributed by Xavier Leroy <Xavier.Leroy@inria.fr>

   and Ulrich Drepper <drepper@cygnus.com>, 1998.

   The GNU C Library is free software; you can redistribute it and/or

   modify it under the terms of the GNU Library General Public License as

   published by the Free Software Foundation; either version 2 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

   Library General Public License for more details.

   You should have received a copy of the GNU Library General Public

   License along with the GNU C Library; see the file COPYING.LIB.  If not,

   write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,

   Boston, MA 02111-1307, USA.  */

#include <errno.h>

#include <pthread.h>

#include <stdlib.h>

#include "internals.h"

#include "queue.h"

#include "spinlock.h"

#include "restart.h"

/*

 * Check whether the calling thread already owns one or more read locks on the

 * specified lock. If so, return a pointer to the read lock info structure

 * corresponding to that lock.

 */

static pthread_readlock_info *

rwlock_is_in_list(pthread_descr self, pthread_rwlock_t *rwlock)

{

  pthread_readlock_info *info;

  for (info = self->p_readlock_list; info != NULL; info = info->pr_next)

    {

      if (info->pr_lock == rwlock)

        return info;

    }

  return NULL;

}

/*

 * Add a new lock to the thread's list of locks for which it has a read lock.

 * A new info node must be allocated for this, which is taken from the thread's

 * free list, or by calling malloc. If malloc fails, a null pointer is

 * returned. Otherwise the lock info structure is initialized and pushed

 * onto the thread's list.

 */

static pthread_readlock_info *

rwlock_add_to_list(pthread_descr self, pthread_rwlock_t *rwlock)

{

  pthread_readlock_info *info = self->p_readlock_free;

  if (info != NULL)

    self->p_readlock_free = info->pr_next;

  else

    info = malloc(sizeof *info);

  if (info == NULL)

    return NULL;

  info->pr_lock_count = 1;

  info->pr_lock = rwlock;

  info->pr_next = self->p_readlock_list;

  self->p_readlock_list = info;

  return info;

}

/*

 * If the thread owns a read lock over the given pthread_rwlock_t,

 * and this read lock is tracked in the thread's lock list,

 * this function returns a pointer to the info node in that list.

 * It also decrements the lock count within that node, and if

 * it reaches zero, it removes the node from the list.

 * If nothing is found, it returns a null pointer.

 */

static pthread_readlock_info *

rwlock_remove_from_list(pthread_descr self, pthread_rwlock_t *rwlock)

{

  pthread_readlock_info **pinfo;

  for (pinfo = &self->p_readlock_list; *pinfo != NULL; pinfo = &(*pinfo)->pr_next)

    {

      if ((*pinfo)->pr_lock == rwlock)

        {

          pthread_readlock_info *info = *pinfo;

          if (--info->pr_lock_count == 0)

            *pinfo = info->pr_next;

          return info;

        }

    }

  return NULL;

}

/*

 * This function checks whether the conditions are right to place a read lock.

 * It returns 1 if so, otherwise zero. The rwlock's internal lock must be

 * locked upon entry.

 */

static int

rwlock_can_rdlock(pthread_rwlock_t *rwlock, int have_lock_already)

{

  /* Can't readlock; it is write locked. */

  if (rwlock->__rw_writer != NULL)

    return 0;

  /* Lock prefers readers; get it. */

  if (rwlock->__rw_kind == PTHREAD_RWLOCK_PREFER_READER_NP)

    return 1;

  /* Lock prefers writers, but none are waiting. */

  if (queue_is_empty(&rwlock->__rw_write_waiting))

    return 1;

  /* Writers are waiting, but this thread already has a read lock */

  if (have_lock_already)

    return 1;

  /* Writers are waiting, and this is a new lock */

  return 0;

}

/*

 * This function helps support brain-damaged recursive read locking

 * semantics required by Unix 98, while maintaining write priority.

 * This basically determines whether this thread already holds a read lock

 * already. It returns 1 if so, otherwise it returns 0.

 *

 * If the thread has any ``untracked read locks'' then it just assumes

 * that this lock is among them, just to be safe, and returns 1.

 *

 * Also, if it finds the thread's lock in the list, it sets the pointer

 * referenced by pexisting to refer to the list entry.

 *

 * If the thread has no untracked locks, and the lock is not found

 * in its list, then it is added to the list. If this fails,

 * then *pout_of_mem is set to 1.

 */

static int

rwlock_have_already(pthread_descr *pself, pthread_rwlock_t *rwlock,

    pthread_readlock_info **pexisting, int *pout_of_mem)

{

  pthread_readlock_info *existing = NULL;

  int out_of_mem = 0, have_lock_already = 0;

  pthread_descr self = *pself;

  if (rwlock->__rw_kind == PTHREAD_RWLOCK_PREFER_WRITER_NP)

    {

      if (!self)

        self = thread_self();

      existing = rwlock_is_in_list(self, rwlock);

      if (existing != NULL || self->p_untracked_readlock_count > 0)

        have_lock_already = 1;

      else

        {

          existing = rwlock_add_to_list(self, rwlock);

          if (existing == NULL)

            out_of_mem = 1;

        }

    }

  *pout_of_mem = out_of_mem;

  *pexisting = existing;

  *pself = self;

  return have_lock_already;

}

int

pthread_rwlock_init (pthread_rwlock_t *rwlock,

                     const pthread_rwlockattr_t *attr)

{

  __pthread_init_lock(&rwlock->__rw_lock);

  rwlock->__rw_readers = 0;

  rwlock->__rw_writer = NULL;

  rwlock->__rw_read_waiting = NULL;

  rwlock->__rw_write_waiting = NULL;

  if (attr == NULL)

    {

      rwlock->__rw_kind = PTHREAD_RWLOCK_DEFAULT_NP;

      rwlock->__rw_pshared = PTHREAD_PROCESS_PRIVATE;

    }

  else

    {

      rwlock->__rw_kind = attr->__lockkind;

      rwlock->__rw_pshared = attr->__pshared;

    }

  return 0;

}

int

pthread_rwlock_destroy (pthread_rwlock_t *rwlock)

{

  int readers;

  _pthread_descr writer;

  __pthread_lock (&rwlock->__rw_lock, NULL);

  readers = rwlock->__rw_readers;

  writer = rwlock->__rw_writer;

  __pthread_unlock (&rwlock->__rw_lock);

  if (readers > 0 || writer != NULL)

    return EBUSY;

  return 0;

}

int

pthread_rwlock_rdlock (pthread_rwlock_t *rwlock)

{

  pthread_descr self = NULL;

  pthread_readlock_info *existing;

  int out_of_mem, have_lock_already;

  have_lock_already = rwlock_have_already(&self, rwlock,

      &existing, &out_of_mem);

  for (;;)

    {

      if (self == NULL)

        self = thread_self ();

      __pthread_lock (&rwlock->__rw_lock, self);

      if (rwlock_can_rdlock(rwlock, have_lock_already))

        break;

      enqueue (&rwlock->__rw_read_waiting, self);

      __pthread_unlock (&rwlock->__rw_lock);

      suspend (self); /* This is not a cancellation point */

    }

  ++rwlock->__rw_readers;

  __pthread_unlock (&rwlock->__rw_lock);

  if (have_lock_already || out_of_mem)

    {

      if (existing != NULL)

        existing->pr_lock_count++;

      else

        self->p_untracked_readlock_count++;

    }

  return 0;

}

int

pthread_rwlock_tryrdlock (pthread_rwlock_t *rwlock)

{

  pthread_descr self = thread_self();

  pthread_readlock_info *existing;

  int out_of_mem, have_lock_already;

  int retval = EBUSY;

  have_lock_already = rwlock_have_already(&self, rwlock,

      &existing, &out_of_mem);

  __pthread_lock (&rwlock->__rw_lock, self);

  /* 0 is passed to here instead of have_lock_already.

     This is to meet Single Unix Spec requirements:

     if writers are waiting, pthread_rwlock_tryrdlock

     does not acquire a read lock, even if the caller has

     one or more read locks already. */

  if (rwlock_can_rdlock(rwlock, 0))

    {

      ++rwlock->__rw_readers;

      retval = 0;

    }

  __pthread_unlock (&rwlock->__rw_lock);

  if (retval == 0)

    {

      if (have_lock_already || out_of_mem)

        {

          if (existing != NULL)

            existing->pr_lock_count++;

          else

            self->p_untracked_readlock_count++;

        }

    }

  return retval;

}

int

pthread_rwlock_wrlock (pthread_rwlock_t *rwlock)

{

  pthread_descr self = thread_self ();

  while(1)

    {

      __pthread_lock (&rwlock->__rw_lock, self);

      if (rwlock->__rw_readers == 0 && rwlock->__rw_writer == NULL)

        {

          rwlock->__rw_writer = self;

          __pthread_unlock (&rwlock->__rw_lock);

          return 0;

        }

      /* Suspend ourselves, then try again */

      enqueue (&rwlock->__rw_write_waiting, self);

      __pthread_unlock (&rwlock->__rw_lock);

      suspend (self); /* This is not a cancellation point */

    }

}

int

pthread_rwlock_trywrlock (pthread_rwlock_t *rwlock)

{

  int result = EBUSY;

  __pthread_lock (&rwlock->__rw_lock, NULL);

  if (rwlock->__rw_readers == 0 && rwlock->__rw_writer == NULL)

    {

      rwlock->__rw_writer = thread_self ();

      result = 0;

    }

  __pthread_unlock (&rwlock->__rw_lock);

  return result;

}

int

pthread_rwlock_unlock (pthread_rwlock_t *rwlock)

{

  pthread_descr torestart;

  pthread_descr th;

  __pthread_lock (&rwlock->__rw_lock, NULL);

  if (rwlock->__rw_writer != NULL)

    {

      /* Unlocking a write lock.  */

      if (rwlock->__rw_writer != thread_self ())

        {

          __pthread_unlock (&rwlock->__rw_lock);

          return EPERM;

        }

      rwlock->__rw_writer = NULL;

      if (rwlock->__rw_kind == PTHREAD_RWLOCK_PREFER_READER_NP

          || (th = dequeue (&rwlock->__rw_write_waiting)) == NULL)

        {

          /* Restart all waiting readers.  */

          torestart = rwlock->__rw_read_waiting;

          rwlock->__rw_read_waiting = NULL;

          __pthread_unlock (&rwlock->__rw_lock);

          while ((th = dequeue (&torestart)) != NULL)

            restart (th);

        }

      else

        {

          /* Restart one waiting writer.  */

          __pthread_unlock (&rwlock->__rw_lock);

          restart (th);

        }

    }

  else

    {

      /* Unlocking a read lock.  */

      if (rwlock->__rw_readers == 0)

        {

          __pthread_unlock (&rwlock->__rw_lock);

          return EPERM;

        }

      --rwlock->__rw_readers;

      if (rwlock->__rw_readers == 0)

        /* Restart one waiting writer, if any.  */

        th = dequeue (&rwlock->__rw_write_waiting);

      else

        th = NULL;

      __pthread_unlock (&rwlock->__rw_lock);

      if (th != NULL)

        restart (th);

      /* Recursive lock fixup */

      if (rwlock->__rw_kind == PTHREAD_RWLOCK_PREFER_WRITER_NP)

        {

          pthread_descr self = thread_self();

          pthread_readlock_info *victim = rwlock_remove_from_list(self, rwlock);

          if (victim != NULL)

            {

              if (victim->pr_lock_count == 0)

                {

                  victim->pr_next = self->p_readlock_free;

                  self->p_readlock_free = victim;

                }

            }

          else

            {

              if (self->p_untracked_readlock_count > 0)

                self->p_untracked_readlock_count--;

            }

        }

    }

  return 0;

}

int

pthread_rwlockattr_init (pthread_rwlockattr_t *attr)

{

  attr->__lockkind = 0;

  attr->__pshared = 0;

  return 0;

}

int

pthread_rwlockattr_destroy (pthread_rwlockattr_t *attr)

{

  return 0;

}

int

pthread_rwlockattr_getpshared (const pthread_rwlockattr_t *attr, int *pshared)

{

  *pshared = attr->__pshared;

  return 0;

}

int

pthread_rwlockattr_setpshared (pthread_rwlockattr_t *attr, int pshared)

{

  if (pshared != PTHREAD_PROCESS_PRIVATE && pshared != PTHREAD_PROCESS_SHARED)

    return EINVAL;

  attr->__pshared = pshared;

  return 0;

}

int

pthread_rwlockattr_getkind_np (const pthread_rwlockattr_t *attr, int *pref)

{

  *pref = attr->__lockkind;

  return 0;

}

int

pthread_rwlockattr_setkind_np (pthread_rwlockattr_t *attr, int pref)

{

  if (pref != PTHREAD_RWLOCK_PREFER_READER_NP

      && pref != PTHREAD_RWLOCK_PREFER_WRITER_NP

      && pref != PTHREAD_RWLOCK_DEFAULT_NP)

    return EINVAL;

  attr->__lockkind = pref;

  return 0;

}