sched/semaphore/sem_setprotocol.c

/****************************************************************************
 * sched/semaphore/sem_setprotocol.c
 *
 * 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/config.h>

#include <assert.h>
#include <errno.h>

#include "semaphore/semaphore.h"

#ifdef CONFIG_PRIORITY_INHERITANCE

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

/****************************************************************************
 * Name: nxsem_set_protocol
 *
 * Description:
 *    Set semaphore protocol attribute.
 *
 *    One particularly important use of this function is when a semaphore
 *    is used for inter-task communication like:
 *
 *      TASK A                 TASK B
 *      nxsem_init(sem, 0, 0);
 *      nxsem_wait(sem);
 *                             nxsem_post(sem);
 *      Awakens as holder
 *
 *    In this case priority inheritance can interfere with the operation of
 *    the semaphore.  The problem is that when TASK A is restarted it is a
 *    holder of the semaphore.  However, it never calls nxsem_post(sem) so it
 *    becomes *permanently* a holder of the semaphore and may have its
 *    priority boosted when any other task tries to acquire the semaphore.
 *
 *    The fix is to call nxsem_set_protocol(SEM_PRIO_NONE) immediately after
 *    the sem_init() call so that there will be no priority inheritance
 *    operations on this semaphore.
 *
 * Input Parameters:
 *    sem      - A pointer to the semaphore whose attributes are to be
 *               modified
 *    protocol - The new protocol to use
 *
 * Returned Value:
 *   This is an internal OS interface and should not be used by applications.
 *   It follows the NuttX internal error return policy:  Zero (OK) is
 *   returned on success.  A negated errno value is returned on failure.
 *
 ****************************************************************************/

int nxsem_set_protocol(FAR sem_t *sem, int protocol)
{
  DEBUGASSERT(sem != NULL);

  switch (protocol & SEM_PRIO_MASK)
    {
      case SEM_PRIO_NONE:

        /* Remove any current holders */

        nxsem_destroyholder(sem);
        break;

      case SEM_PRIO_INHERIT:

        break;

      case SEM_PRIO_PROTECT:

        /* Not yet supported */

        return -ENOTSUP;

      default:
        return -EINVAL;
    }

  sem->flags = protocol;
  return OK;
}

#endif /* CONFIG_PRIORITY_INHERITANCE */