/****************************************************************************
* sched/semaphore/sem_post.c
*
* Copyright (C) 2007-2009, 2012-2013, 2016 Gregory Nutt. All rights reserved.
* Author: Gregory Nutt <gnutt@nuttx.org>
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in
* the documentation and/or other materials provided with the
* distribution.
* 3. Neither the name NuttX nor the names of its contributors may be
* used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
* FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
* COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
* BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
* OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
* AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
* ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*
****************************************************************************/
/****************************************************************************
* Included Files
****************************************************************************/
#include <nuttx/config.h>
#include <limits.h>
#include <semaphore.h>
#include <errno.h>
#include <sched.h>
#include <nuttx/irq.h>
#include <nuttx/arch.h>
#include "sched/sched.h"
#include "semaphore/semaphore.h"
/****************************************************************************
* Public Functions
****************************************************************************/
/****************************************************************************
* Name: sem_post
*
* Description:
* When a task has finished with a semaphore, it will call sem_post().
* This function unlocks the semaphore referenced by sem by performing the
* semaphore unlock operation on that semaphore.
*
* If the semaphore value resulting from this operation is positive, then
* no tasks were blocked waiting for the semaphore to become unlocked; the
* semaphore is simply incremented.
*
* If the value of the semaphore resulting from this operation is zero,
* then one of the tasks blocked waiting for the semaphore shall be
* allowed to return successfully from its call to sem_wait().
*
* Parameters:
* sem - Semaphore descriptor
*
* Return Value:
* 0 (OK) or -1 (ERROR) if unsuccessful
*
* Assumptions:
* This function may be called from an interrupt handler.
*
****************************************************************************/
int sem_post(FAR sem_t *sem)
{
FAR struct tcb_s *stcb = NULL;
irqstate_t flags;
int ret = ERROR;
/* Make sure we were supplied with a valid semaphore. */
if (sem)
{
/* The following operations must be performed with interrupts
* disabled because sem_post() may be called from an interrupt
* handler.
*/
flags = enter_critical_section();
/* Perform the semaphore unlock operation, releasing this task as a
* holder then also incrementing the count on the semaphore.
*
* NOTE: When semaphores are used for signaling purposes, the holder
* of the semaphore may not be this thread! In this case,
* sem_releaseholder() will do nothing.
*
* In the case of a mutex this could be simply resolved since there is
* only one holder but for the case of counting semaphores, there may
* be many holders and if the holder is not this thread, then it is
* not possible to know which thread/holder should be released.
*
* For this reason, it is recommended that priority inheritance be
* disabled via sem_setprotocol(SEM_PRIO_NONE) when the semahore is
* initialixed if the semaphore is to used for signaling purposes.
*/
ASSERT(sem->semcount < SEM_VALUE_MAX);
sem_releaseholder(sem);
sem->semcount++;
#ifdef CONFIG_PRIORITY_INHERITANCE
/* Don't let any unblocked tasks run until we complete any priority
* restoration steps. Interrupts are disabled, but we do not want
* the head of the read-to-run list to be modified yet.
*
* NOTE: If this sched_lock is called from an interrupt handler, it
* will do nothing.
*/
sched_lock();
#endif
/* If the result of of semaphore unlock is non-positive, then
* there must be some task waiting for the semaphore.
*/
if (sem->semcount <= 0)
{
/* Check if there are any tasks in the waiting for semaphore
* task list that are waiting for this semaphore. This is a
* prioritized list so the first one we encounter is the one
* that we want.
*/
for (stcb = (FAR struct tcb_s *)g_waitingforsemaphore.head;
(stcb && stcb->waitsem != sem);
stcb = stcb->flink);
if (stcb != NULL)
{
/* The task will be the new holder of the semaphore when
* it is awakened.
*/
sem_addholder_tcb(stcb, sem);
/* It is, let the task take the semaphore */
stcb->waitsem = NULL;
/* Restart the waiting task. */
up_unblock_task(stcb);
}
}
/* Check if we need to drop the priority of any threads holding
* this semaphore. The priority could have been boosted while they
* held the semaphore.
*/
#ifdef CONFIG_PRIORITY_INHERITANCE
sem_restorebaseprio(stcb, sem);
sched_unlock();
#endif
ret = OK;
/* Interrupts may now be enabled. */
leave_critical_section(flags);
}
else
{
set_errno(EINVAL);
}
return ret;
}