/****************************************************************************
* sched/sem_wait.c
*
* Copyright (C) 2007-2013 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 <stdbool.h>
#include <semaphore.h>
#include <errno.h>
#include <assert.h>
#include <nuttx/arch.h>
#include "os_internal.h"
#include "sem_internal.h"
/****************************************************************************
* Pre-processor Definitions
****************************************************************************/
/****************************************************************************
* Private Type Declarations
****************************************************************************/
/****************************************************************************
* Global Variables
****************************************************************************/
/****************************************************************************
* Private Variables
****************************************************************************/
/****************************************************************************
* Private Functions
****************************************************************************/
/****************************************************************************
* Public Functions
****************************************************************************/
/****************************************************************************
* Name: sem_wait
*
* Description:
* This function attempts to lock the semaphore referenced by 'sem'. If
* the semaphore value is (<=) zero, then the calling task will not return
* until it successfully acquires the lock.
*
* Parameters:
* sem - Semaphore descriptor.
*
* Return Value:
* 0 (OK), or -1 (ERROR) is unsuccessful
* If this function returns -1 (ERROR), then the cause of the failure will
* be reported in 'errno' as:
* - EINVAL: Invalid attempt to get the semaphore
* - EINTR: The wait was interrupted by the receipt of a signal.
*
* Assumptions:
*
****************************************************************************/
int sem_wait(FAR sem_t *sem)
{
FAR struct tcb_s *rtcb = (FAR struct tcb_s*)g_readytorun.head;
irqstate_t saved_state;
int ret = ERROR;
/* This API should not be called from interrupt handlers */
DEBUGASSERT(up_interrupt_context() == false)
/* Assume any errors reported are due to invalid arguments. */
errno = EINVAL;
if (sem)
{
/* The following operations must be performed with interrupts
* disabled because sem_post() may be called from an interrupt
* handler.
*/
saved_state = irqsave();
/* Check if the lock is available */
if (sem->semcount > 0)
{
/* It is, let the task take the semaphore. */
sem->semcount--;
sem_addholder(sem);
rtcb->waitsem = NULL;
ret = OK;
}
/* The semaphore is NOT available, We will have to block the
* current thread of execution.
*/
else
{
/* First, verify that the task is not already waiting on a
* semaphore
*/
ASSERT(rtcb->waitsem == NULL);
/* Handle the POSIX semaphore (but don't set the owner yet) */
sem->semcount--;
/* Save the waited on semaphore in the TCB */
rtcb->waitsem = sem;
/* If priority inheritance is enabled, then check the priority of
* the holder of the semaphore.
*/
#ifdef CONFIG_PRIORITY_INHERITANCE
/* Disable context switching. The following operations must be
* atomic with regard to the scheduler.
*/
sched_lock();
/* Boost the priority of any threads holding a count on the
* semaphore.
*/
sem_boostpriority(sem);
#endif
/* Add the TCB to the prioritized semaphore wait queue */
errno = 0;
up_block_task(rtcb, TSTATE_WAIT_SEM);
/* When we resume at this point, either (1) the semaphore has been
* assigned to this thread of execution, or (2) the semaphore wait
* has been interrupted by a signal or a timeout. We can detect these
* latter cases be examining the errno value.
*
* In the event that the semaphore wait was interrupted by a signal or
* a timeout, certain semaphore clean-up operations have already been
* performed (see sem_waitirq.c). Specifically:
*
* - sem_canceled() was called to restore the priority of all threads
* that hold a reference to the semaphore,
* - The semaphore count was decremented, and
* - tcb->waitsem was nullifed.
*
* It is necesaary to do these things in sem_waitirq.c because a long
* time may elapse between the time that the signal was issued and
* this thread is awakened and this leaves a door open to several
* race conditions.
*/
if (errno != EINTR && errno != ETIMEDOUT)
{
/* Not awakened by a signal or a timeout... We hold the semaphore */
sem_addholder(sem);
ret = OK;
}
#ifdef CONFIG_PRIORITY_INHERITANCE
sched_unlock();
#endif
}
/* Interrupts may now be enabled. */
irqrestore(saved_state);
}
return ret;
}
