sched/semaphore/sem_reset.c

/****************************************************************************
 * sched/semaphore/sem_reset.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/config.h>

#include <sched.h>
#include <assert.h>

#include <nuttx/irq.h>

#include "semaphore/semaphore.h"

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

/****************************************************************************
 * Name: nxsem_reset
 *
 * Description:
 *   Reset a semaphore count to a specific value.  This is similar to part
 *   of the operation of nxsem_init().  But nxsem_reset() may need to wake up
 *   tasks waiting on a count.  This kind of operation is sometimes required
 *   within the OS (only) for certain error handling conditions.
 *
 *   Mutex is simply posted until it is not blocking any tasks. If the
 *   requested count is 0, a single running holder is left. If the requested
 *   count is 1, the mutex is set to "reset". Other requested counts are not
 *   allowed for mutex.
 *
 * Input Parameters:
 *   sem   - Semaphore descriptor to be reset
 *   count - The requested semaphore count
 *
 * Returned Value:
 *   This is an internal OS interface, not available to applications, and
 *   hence follows the NuttX internal error return policy:  Zero (OK) is
 *   returned on success.  A negated errno value is returned on failure.
 *
 ****************************************************************************/

int nxsem_reset(FAR sem_t *sem, int16_t count)
{
  irqstate_t flags;
  int32_t semcount;

  DEBUGASSERT(sem != NULL && count >= 0);

  /* Don't allow any context switches that may result from the following
   * nxsem_post() operations.
   */

  sched_lock();

  /* Prevent any access to the semaphore by interrupt handlers while we are
   * performing this operation.
   */

  flags = enter_critical_section();

  if (NXSEM_IS_MUTEX(sem))
    {
      /* Support only resetting mutex by removing one waiter */

      DEBUGASSERT(count == 1);

      /* Post the mutex once with holder value set to RESET | BLOCKS
       * so we know that it is ok in this case to call the post from
       * another thread.
       */

      atomic_set(NXSEM_MHOLDER(sem),
                 NXSEM_MRESET | NXSEM_MBLOCKING_BIT);

      if (!dq_empty(SEM_WAITLIST(sem)))
        {
          DEBUGVERIFY(nxsem_post(sem));
        }
      else
        {
          atomic_set(NXSEM_MHOLDER(sem), NXSEM_MRESET);
        }
    }
  else
    {
      /* A negative count indicates that the negated number of threads are
       * waiting to take a count from the semaphore.  Loop here, handing
       * out counts to any waiting threads.
       */

      while (atomic_read(NXSEM_COUNT(sem)) < 0 && count > 0)
        {
          /* Give out one counting, waking up one of the waiting threads
           * and, perhaps, kicking off a lot of priority inheritance
           * logic (REVISIT).
           */

          DEBUGVERIFY(nxsem_post(sem));
          count--;
        }

      /* We exit the above loop with either (1) no threads waiting for the
       * (i.e., with sem->semcount >= 0).  In this case, 'count' holds the
       * the new value of the semaphore count.  OR (2) with threads still
       * waiting but all of the semaphore counts exhausted:  The current
       * value of sem->semcount is already correct in this case.
       */

      semcount = atomic_read(NXSEM_COUNT(sem));
      do
        {
          if (semcount < 0)
            {
              break;
            }
        }
      while (!atomic_try_cmpxchg_release(NXSEM_COUNT(sem), &semcount,
                                         count));
    }

  /* Allow any pending context switches to occur now */

  leave_critical_section(flags);
  sched_unlock();
  return OK;
}