nuttx/mm/iob/iob_alloc.c

/****************************************************************************
 * mm/iob/iob_alloc.c
 *
 *   Copyright (C) 2014, 2016-2017 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 <semaphore.h>
#include <assert.h>
#include <errno.h>

#include <nuttx/irq.h>
#include <nuttx/arch.h>
#include <nuttx/sched.h>
#include <nuttx/mm/iob.h>

#include "iob.h"

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

/****************************************************************************
 * Name: iob_alloc_committed
 *
 * Description:
 *   Allocate an I/O buffer by taking the buffer at the head of the committed
 *   list.
 *
 ****************************************************************************/

static FAR struct iob_s *iob_alloc_committed(void)
{
  FAR struct iob_s *iob = NULL;
  irqstate_t flags;

  /* We don't know what context we are called from so we use extreme measures
   * to protect the committed list:  We disable interrupts very briefly.
   */

  flags = enter_critical_section();

  /* Take the I/O buffer from the head of the committed list */

  iob = g_iob_committed;
  if (iob != NULL)
    {
      /* Remove the I/O buffer from the committed list */

      g_iob_committed = iob->io_flink;

      /* Put the I/O buffer in a known state */

      iob->io_flink  = NULL; /* Not in a chain */
      iob->io_len    = 0;    /* Length of the data in the entry */
      iob->io_offset = 0;    /* Offset to the beginning of data */
      iob->io_pktlen = 0;    /* Total length of the packet */
    }

  leave_critical_section(flags);
  return iob;
}

/****************************************************************************
 * Name: iob_allocwait
 *
 * Description:
 *   Allocate an I/O buffer, waiting if necessary.  This function cannot be
 *   called from any interrupt level logic.
 *
 ****************************************************************************/

static FAR struct iob_s *iob_allocwait(bool throttled)
{
  FAR struct iob_s *iob;
  irqstate_t flags;
  FAR sem_t *sem;
  int ret = OK;

#if CONFIG_IOB_THROTTLE > 0
  /* Select the semaphore count to check. */

  sem = (throttled ? &g_throttle_sem : &g_iob_sem);
#else
  sem = &g_iob_sem;
#endif

  /* The following must be atomic; interrupt must be disabled so that there
   * is no conflict with interrupt level I/O buffer allocations.  This is
   * not as bad as it sounds because interrupts will be re-enabled while
   * we are waiting for I/O buffers to become free.
   */

  flags = enter_critical_section();

  /* Try to get an I/O buffer.  If successful, the semaphore count will be
   * decremented atomically.
   */

  iob = iob_tryalloc(throttled);
  while (ret == OK && iob == NULL)
    {
      /* If not successful, then the semaphore count was less than or equal
       * to zero (meaning that there are no free buffers).  We need to wait
       * for an I/O buffer to be released and placed in the committed
       * list.
       */

      ret = sem_wait(sem);
      if (ret < 0)
        {
          int errcode = get_errno();

          /* EINTR is not an error!  EINTR simply means that we were
           * awakened by a signal and we should try again.
           *
           * REVISIT:  Many end-user interfaces are required to return with
           * an error if EINTR is set.  Most uses of this function are in
           * internal, non-user logic.  But are there cases where the error
           * should be returned.
           */

          if (errcode == EINTR)
            {
              /* Force a success indication so that we will continue looping. */

              ret = 0;
            }
          else
            {
              /* Stop the loop and return a error */

              DEBUGASSERT(errcode > 0);
              ret = -errcode;
            }
        }
      else
        {
          /* When we wake up from wait successfully, an I/O buffer was
           * freed and we hold a count for one IOB.  Unless somehting
           * failed, we should have an IOB waiting for us in the
           * committed list.
           */

          iob = iob_alloc_committed();
          DEBUGASSERT(iob != NULL);

          if (iob == NULL)
            {
              /* This should not fail, but we allow for that possibility to
               * handle any potential, non-obvious race condition.  Perhaps
               * the free IOB ended up in the g_iob_free list?
               *
               * We need release our count so that it is available to
               * iob_tryalloc(), perhaps allowing another thread to take our
               * count.  In that event, iob_tryalloc() will fail above and
               * we will have to wait again.
               */

              sem_post(sem);
              iob = iob_tryalloc(throttled);
            }
        }
    }

  leave_critical_section(flags);
  return iob;
}

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

/****************************************************************************
 * Name: iob_alloc
 *
 * Description:
 *   Allocate an I/O buffer by taking the buffer at the head of the free list.
 *
 ****************************************************************************/

FAR struct iob_s *iob_alloc(bool throttled)
{
  /* Were we called from the interrupt level? */

  if (up_interrupt_context() || sched_idletask())
    {
      /* Yes, then try to allocate an I/O buffer without waiting */

      return iob_tryalloc(throttled);
    }
  else
    {
      /* Then allocate an I/O buffer, waiting as necessary */

      return iob_allocwait(throttled);
    }
}

/****************************************************************************
 * Name: iob_tryalloc
 *
 * Description:
 *   Try to allocate an I/O buffer by taking the buffer at the head of the
 *   free list without waiting for a buffer to become free.
 *
 ****************************************************************************/

FAR struct iob_s *iob_tryalloc(bool throttled)
{
  FAR struct iob_s *iob;
  irqstate_t flags;
#if CONFIG_IOB_THROTTLE > 0
  FAR sem_t *sem;
#endif

#if CONFIG_IOB_THROTTLE > 0
  /* Select the semaphore count to check. */

  sem = (throttled ? &g_throttle_sem : &g_iob_sem);
#endif

  /* We don't know what context we are called from so we use extreme measures
   * to protect the free list:  We disable interrupts very briefly.
   */

  flags = enter_critical_section();

#if CONFIG_IOB_THROTTLE > 0
  /* If there are free I/O buffers for this allocation */

  if (sem->semcount > 0)
#endif
    {
      /* Take the I/O buffer from the head of the free list */

      iob = g_iob_freelist;
      if (iob != NULL)
        {
          /* Remove the I/O buffer from the free list and decrement the
           * counting semaphore(s) that tracks the number of available
           * IOBs.
           */

          g_iob_freelist = iob->io_flink;

          /* Take a semaphore count.  Note that we cannot do this in
           * in the orthodox way by calling sem_wait() or sem_trywait()
           * because this function may be called from an interrupt
           * handler. Fortunately we know at at least one free buffer
           * so a simple decrement is all that is needed.
           */

          g_iob_sem.semcount--;
          DEBUGASSERT(g_iob_sem.semcount >= 0);

#if CONFIG_IOB_THROTTLE > 0
          /* The throttle semaphore is a little more complicated because
           * it can be negative!  Decrementing is still safe, however.
           */

          g_throttle_sem.semcount--;
          DEBUGASSERT(g_throttle_sem.semcount >= -CONFIG_IOB_THROTTLE);
#endif
          leave_critical_section(flags);

          /* Put the I/O buffer in a known state */

          iob->io_flink  = NULL; /* Not in a chain */
          iob->io_len    = 0;    /* Length of the data in the entry */
          iob->io_offset = 0;    /* Offset to the beginning of data */
          iob->io_pktlen = 0;    /* Total length of the packet */
          return iob;
        }
    }

  leave_critical_section(flags);
  return NULL;
}