incubator-nuttx/sched/semaphore/sem_trywait.c

/****************************************************************************
 * sched/semaphore/sem_trywait.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 <stdbool.h>
#include <sched.h>
#include <assert.h>
#include <errno.h>

#include <nuttx/init.h>
#include <nuttx/irq.h>
#include <nuttx/arch.h>

#include "sched/sched.h"
#include "semaphore/semaphore.h"

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

/****************************************************************************
 * Name: nxsem_trywait_slow
 *
 * Description:
 *   This function locks the specified semaphore in slow mode.
 *
 * Input Parameters:
 *   sem - the semaphore descriptor
 *
 * Returned Value:
 *
 *     EINVAL - Invalid attempt to get the semaphore
 *     EAGAIN - The semaphore is not available.
 *
 * Assumptions:
 *
 ****************************************************************************/

static int nxsem_trywait_slow(FAR sem_t *sem)
{
  FAR struct tcb_s *rtcb;
  irqstate_t flags;
  short semcount;
  int ret;

  /* The following operations must be performed with interrupts disabled
   * because sem_post() may be called from an interrupt handler.
   */

  flags = enter_critical_section();
  rtcb = this_task();

  /* If the semaphore is available, give it to the requesting task */

  do
    {
      semcount = atomic_load(NXSEM_COUNT(sem));
      if (semcount <= 0)
        {
          leave_critical_section(flags);
          return -EAGAIN;
        }
    }
  while (!atomic_compare_exchange_weak_explicit(NXSEM_COUNT(sem),
                                                &semcount, semcount - 1,
                                                memory_order_acquire,
                                                memory_order_relaxed));

  /* It is, let the task take the semaphore */

  ret = nxsem_protect_wait(sem);
  if (ret < 0)
    {
      atomic_fetch_add(NXSEM_COUNT(sem), 1);
      leave_critical_section(flags);
      return ret;
    }

  nxsem_add_holder(sem);
  rtcb->waitobj = NULL;
  ret = OK;

  /* Interrupts may now be enabled. */

  leave_critical_section(flags);
  return ret;
}

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

/****************************************************************************
 * Name: nxsem_trywait
 *
 * Description:
 *   This function locks the specified semaphore only if the semaphore is
 *   currently not locked.  In either case, the call returns without
 *   blocking.
 *
 * Input Parameters:
 *   sem - the semaphore descriptor
 *
 * 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.
 *   Possible returned errors:
 *
 *     EINVAL - Invalid attempt to get the semaphore
 *     EAGAIN - The semaphore is not available.
 *
 * Assumptions:
 *
 ****************************************************************************/

int nxsem_trywait(FAR sem_t *sem)
{
  /* This API should not be called from the idleloop */

  DEBUGASSERT(sem != NULL);
  DEBUGASSERT(!OSINIT_IDLELOOP() || !sched_idletask() ||
              up_interrupt_context());

  /* If this is a mutex, we can try to get the mutex in fast mode,
   * else try to get it in slow mode.
   */

#if !defined(CONFIG_PRIORITY_INHERITANCE) && !defined(CONFIG_PRIORITY_PROTECT)
  if (sem->flags & SEM_TYPE_MUTEX)
    {
      short old = 1;
      if (atomic_compare_exchange_weak_explicit(NXSEM_COUNT(sem), &old, 0,
                                                memory_order_acquire,
                                                memory_order_relaxed))
        {
          return OK;
        }

      return -EAGAIN;
    }
#endif

  return nxsem_trywait_slow(sem);
}