nuttx/sched/semaphore/sem_rw.c

/****************************************************************************
 * sched/semaphore/sem_rw.c
 *
 * SPDX-License-Identifier: Apache-2.0
 *
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.  The
 * ASF licenses this file to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance with the
 * License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
 * License for the specific language governing permissions and limitations
 * under the License.
 *
 ****************************************************************************/

/****************************************************************************
 * Included Files
 ****************************************************************************/

#include <nuttx/rwsem.h>
#include <nuttx/sched.h>
#include <assert.h>

/****************************************************************************
 * Private Functions
 ****************************************************************************/

static inline void up_wait(FAR rw_semaphore_t *rwsem)
{
  int i;

  for (i = 0; i < rwsem->waiter; i++)
    {
      /* If there are some waiter for unlock, then post the lock wait queue.
       */

      nxsem_post(&rwsem->waiting);
    }
}

/****************************************************************************
 * Public Functions
 ****************************************************************************/

/****************************************************************************
 * Name: down_read_trylock
 *
 * Description:
 *   Acquire a read lock on a read-write-lock object.
 *
 * Input Parameters:
 *   rwsem  - Pointer to the read-write-lock descriptor.
 *
 * Returned Value:
 *   Return 1 if successful, 0 if failed
 *
 ****************************************************************************/

int down_read_trylock(FAR rw_semaphore_t *rwsem)
{
  int ret = 1;

  nxmutex_lock(&rwsem->protected);

  /* if the write lock is already held by oneself and since the write lock
   * can be recursively held, so, this operation can be converted to a write
   * lock to avoid deadlock.
   */

  if (rwsem->holder == _SCHED_GETTID())
    {
      rwsem->writer++;
    }
  else if (rwsem->writer > 0)
    {
      ret = 0;
    }
  else
    {
      /* In a scenario where there is no write lock, we just need to
       * make the read base +1.
       */

      rwsem->reader++;
    }

  nxmutex_unlock(&rwsem->protected);
  return ret;
}

/****************************************************************************
 * Name: down_read
 *
 * Description:
 *   Acquire a read lock on a read-write-lock object.
 *
 * Input Parameters:
 *   rwsem  - Pointer to the read-write-lock descriptor.
 *
 ****************************************************************************/

void down_read(FAR rw_semaphore_t *rwsem)
{
  /* we have to check if there is a write-lock scenario, if there is then we
   * block and wait for the write-lock to be unlocked.
   */

  nxmutex_lock(&rwsem->protected);

  /* if the write lock is already held by oneself and since the write lock
   * can be recursively held, so, this operation can be converted to a write
   * lock to avoid deadlock.
   */

  if (rwsem->holder == _SCHED_GETTID())
    {
      rwsem->writer++;
    }
  else
    {
      while (rwsem->writer > 0)
        {
          rwsem->waiter++;
          nxmutex_unlock(&rwsem->protected);
          nxsem_wait(&rwsem->waiting);
          nxmutex_lock(&rwsem->protected);
          rwsem->waiter--;
        }

      /* In a scenario where there is no write lock, we just need to make the
       * read base +1.
       */

      rwsem->reader++;
    }

  nxmutex_unlock(&rwsem->protected);
}

/****************************************************************************
 * Name: up_read
 *
 * Description:
 *   Unlock a read lock on a read-write-lock object.
 *
 * Input Parameters:
 *   rwsem  - Pointer to the read-write-lock descriptor.
 *
 ****************************************************************************/

void up_read(FAR rw_semaphore_t *rwsem)
{
  nxmutex_lock(&rwsem->protected);

  /* when releasing a read lock and holder is oneself, the read lock is a
   * write lock that has been converted, so it should be released according
   * to the procedures for releasing a write lock.
   */

  if (rwsem->holder == _SCHED_GETTID())
    {
      if (--rwsem->writer <= 0)
        {
          rwsem->holder = RWSEM_NO_HOLDER;
          up_wait(rwsem);
        }
    }
  else
    {
      DEBUGASSERT(rwsem->reader > 0);

      if (--rwsem->reader <= 0)
        {
          up_wait(rwsem);
        }
    }

  nxmutex_unlock(&rwsem->protected);
}

/****************************************************************************
 * Name: down_write_trylock
 *
 * Description:
 *   Acquire a write lock on a read-write-lock object.
 *
 * Input Parameters:
 *   rwsem  - Pointer to the read-write-lock descriptor.
 *
 * Returned Value:
 *   Return 1 if successful, 0 if failed
 *
 ****************************************************************************/

int down_write_trylock(FAR rw_semaphore_t *rwsem)
{
  pid_t tid = _SCHED_GETTID();
  int ret = 1;

  nxmutex_lock(&rwsem->protected);

  if (rwsem->reader > 0 || (rwsem->writer > 0 && tid != rwsem->holder))
    {
      nxmutex_unlock(&rwsem->protected);
      ret = 0;
    }
  else
    {
      /* The check passes, then we just need the writer reference + 1 */

      rwsem->writer++;
      rwsem->holder = tid;

      nxmutex_unlock(&rwsem->protected);
    }

  return ret;
}

/****************************************************************************
 * Name: down_write
 *
 * Description:
 *   Acquire a write lock on a read-write-lock object.
 *
 * Input Parameters:
 *   rwsem  - Pointer to the read-write-lock descriptor.
 *
 ****************************************************************************/

void down_write(FAR rw_semaphore_t *rwsem)
{
  pid_t tid = _SCHED_GETTID();

  nxmutex_lock(&rwsem->protected);

  while (rwsem->reader > 0 || (rwsem->writer > 0 && rwsem->holder != tid))
    {
      rwsem->waiter++;
      nxmutex_unlock(&rwsem->protected);
      nxsem_wait(&rwsem->waiting);
      nxmutex_lock(&rwsem->protected);
      rwsem->waiter--;
    }

  /* The check passes, then we just need the writer reference + 1 */

  rwsem->writer++;
  rwsem->holder = tid;

  nxmutex_unlock(&rwsem->protected);
}

/****************************************************************************
 * Name: up_write
 *
 * Description:
 *   Unlock a write lock on a read-write-lock object.
 *
 * Input Parameters:
 *   rwsem  - Pointer to the read-write-lock descriptor.
 *
 ****************************************************************************/

void up_write(FAR rw_semaphore_t *rwsem)
{
  nxmutex_lock(&rwsem->protected);

  DEBUGASSERT(rwsem->writer > 0);
  DEBUGASSERT(rwsem->holder == _SCHED_GETTID());

  if (--rwsem->writer <= 0)
    {
      rwsem->holder = RWSEM_NO_HOLDER;
      up_wait(rwsem);
    }

  nxmutex_unlock(&rwsem->protected);
}

/****************************************************************************
 * Name: downgrade_write
 *
 * Description:
 *   Downgrade write lock to read lock on a read-write-lock object.
 *
 * Input Parameters:
 *   rwsem  - Pointer to the read-write-lock descriptor.
 *
 ****************************************************************************/

void downgrade_write(FAR rw_semaphore_t *rwsem)
{
  nxmutex_lock(&rwsem->protected);

  DEBUGASSERT(rwsem->writer == 1);
  DEBUGASSERT(rwsem->reader == 0);
  DEBUGASSERT(rwsem->holder == _SCHED_GETTID());

  rwsem->writer = 0;
  rwsem->reader++;
  rwsem->holder = RWSEM_NO_HOLDER;

  up_wait(rwsem);
  nxmutex_unlock(&rwsem->protected);
}

/****************************************************************************
 * Name: init_rwsem
 *
 * Description:
 *   Initialize a read-write-lock object, setting its initial state.
 *
 * Input Parameters:
 *   rwsem  - Pointer to the read-write-lock descriptor.
 *
 * Returned Value:
 *   It follows the NuttX internal error return policy: Zero (OK) is
 *   returned on success. A negated errno value is returned on failure.
 *
 ****************************************************************************/

int init_rwsem(FAR rw_semaphore_t *rwsem)
{
  /* Initialize structure information */

  nxmutex_init(&rwsem->protected);
  nxsem_init(&rwsem->waiting, 0, 0);

  rwsem->reader = 0;
  rwsem->writer = 0;
  rwsem->waiter = 0;
  rwsem->holder = RWSEM_NO_HOLDER;

  return OK;
}

/****************************************************************************
 * Name: destroy_rwsem
 *
 * Description:
 *   Destroy a read-write-lock object, freeing any resources associated with
 *   it.
 *
 * Input Parameters:
 *   rwsem - Pointer to the read-write-lock descriptor.
 *
 ****************************************************************************/

void destroy_rwsem(FAR rw_semaphore_t *rwsem)
{
  /* Need to check if there is still an unlocked or waiting state */

  DEBUGASSERT(rwsem->waiter == 0 && rwsem->reader == 0 &&
              rwsem->writer == 0 && rwsem->holder == RWSEM_NO_HOLDER);

  nxmutex_destroy(&rwsem->protected);
  nxsem_destroy(&rwsem->waiting);
}