incubator-nuttx/mm/iob/iob.h

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/****************************************************************************
* mm/iob/iob.h
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*
* 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
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*
* http://www.apache.org/licenses/LICENSE-2.0
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*
* 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.
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*
****************************************************************************/
#ifndef __MM_IOB_IOB_H
#define __MM_IOB_IOB_H
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/****************************************************************************
* Included Files
****************************************************************************/
#include <nuttx/config.h>
#include <debug.h>
#include <nuttx/mm/iob.h>
#include <nuttx/semaphore.h>
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#ifdef CONFIG_MM_IOB
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/****************************************************************************
* Pre-processor Definitions
****************************************************************************/
#define ROUNDUP(x, y) (((x) + (y) - 1) / (y) * (y))
#if defined(CONFIG_DEBUG_FEATURES) && defined(CONFIG_IOB_DEBUG)
# define ioberr _err
# define iobwarn _warn
# define iobinfo _info
#else
# define ioberr _none
# define iobwarn _none
# define iobinfo _none
#endif /* CONFIG_DEBUG_FEATURES && CONFIG_IOB_DEBUG */
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/****************************************************************************
* Public Data
****************************************************************************/
/* A list of all free, unallocated I/O buffers */
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extern FAR struct iob_s *g_iob_freelist;
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There can be a failure in IOB allocation to some asynchronous behavior caused by the use of sem_post(). Consider this scenario: Task A holds an IOB.  There are no further IOBs.  The value of semcount is zero. Task B calls iob_alloc().  Since there are not IOBs, it calls sem_wait().  The v alue of semcount is now -1. Task A frees the IOB.  iob_free() adds the IOB to the free list and calls sem_post() this makes Task B ready to run and sets semcount to zero NOT 1.  There is one IOB in the free list and semcount is zero.  When Task B wakes up it would increment the sem_count back to the correct value. But an interrupt or another task runs occurs before Task B executes.  The interrupt or other tak takes the IOB off of the free list and decrements the semcount.  But since semcount is then < 0, this causes the assertion because that is an invalid state in the interrupt handler. So I think that the root cause is that there the asynchrony between incrementing the semcount. This change separates the list of IOBs: Currently there is only a free list of IOBs. The problem, I believe, is because of asynchronies due sem_post() post cause the semcount and the list content to become out of sync. This change adds a new 'committed' list: When there is a task waiting for an IOB, it will go into the committed list rather than the free list before the semaphore is posted. On the waiting side, when awakened from the semaphore wait, it will expect to find its IOB in the committed list, rather than free list. In this way, the content of the free list and the value of the semaphore count always remain in sync.
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/* A list of I/O buffers that are committed for allocation */
extern FAR struct iob_s *g_iob_committed;
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#if CONFIG_IOB_NCHAINS > 0
There can be a failure in IOB allocation to some asynchronous behavior caused by the use of sem_post(). Consider this scenario: Task A holds an IOB.  There are no further IOBs.  The value of semcount is zero. Task B calls iob_alloc().  Since there are not IOBs, it calls sem_wait().  The v alue of semcount is now -1. Task A frees the IOB.  iob_free() adds the IOB to the free list and calls sem_post() this makes Task B ready to run and sets semcount to zero NOT 1.  There is one IOB in the free list and semcount is zero.  When Task B wakes up it would increment the sem_count back to the correct value. But an interrupt or another task runs occurs before Task B executes.  The interrupt or other tak takes the IOB off of the free list and decrements the semcount.  But since semcount is then < 0, this causes the assertion because that is an invalid state in the interrupt handler. So I think that the root cause is that there the asynchrony between incrementing the semcount. This change separates the list of IOBs: Currently there is only a free list of IOBs. The problem, I believe, is because of asynchronies due sem_post() post cause the semcount and the list content to become out of sync. This change adds a new 'committed' list: When there is a task waiting for an IOB, it will go into the committed list rather than the free list before the semaphore is posted. On the waiting side, when awakened from the semaphore wait, it will expect to find its IOB in the committed list, rather than free list. In this way, the content of the free list and the value of the semaphore count always remain in sync.
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/* A list of all free, unallocated I/O buffer queue containers */
extern FAR struct iob_qentry_s *g_iob_freeqlist;
There can be a failure in IOB allocation to some asynchronous behavior caused by the use of sem_post(). Consider this scenario: Task A holds an IOB.  There are no further IOBs.  The value of semcount is zero. Task B calls iob_alloc().  Since there are not IOBs, it calls sem_wait().  The v alue of semcount is now -1. Task A frees the IOB.  iob_free() adds the IOB to the free list and calls sem_post() this makes Task B ready to run and sets semcount to zero NOT 1.  There is one IOB in the free list and semcount is zero.  When Task B wakes up it would increment the sem_count back to the correct value. But an interrupt or another task runs occurs before Task B executes.  The interrupt or other tak takes the IOB off of the free list and decrements the semcount.  But since semcount is then < 0, this causes the assertion because that is an invalid state in the interrupt handler. So I think that the root cause is that there the asynchrony between incrementing the semcount. This change separates the list of IOBs: Currently there is only a free list of IOBs. The problem, I believe, is because of asynchronies due sem_post() post cause the semcount and the list content to become out of sync. This change adds a new 'committed' list: When there is a task waiting for an IOB, it will go into the committed list rather than the free list before the semaphore is posted. On the waiting side, when awakened from the semaphore wait, it will expect to find its IOB in the committed list, rather than free list. In this way, the content of the free list and the value of the semaphore count always remain in sync.
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/* A list of I/O buffer queue containers that are committed for allocation */
extern FAR struct iob_qentry_s *g_iob_qcommitted;
#endif
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/* Counting semaphores that tracks the number of free IOBs/qentries */
extern sem_t g_iob_sem; /* Counts free I/O buffers */
#if CONFIG_IOB_THROTTLE > 0
extern sem_t g_throttle_sem; /* Counts available I/O buffers when throttled */
#endif
#if CONFIG_IOB_NCHAINS > 0
extern sem_t g_qentry_sem; /* Counts free I/O buffer queue containers */
#endif
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/****************************************************************************
* Public Function Prototypes
****************************************************************************/
/****************************************************************************
* Name: iob_alloc_qentry
*
* Description:
* Allocate an I/O buffer chain container by taking the buffer at the head
* of the free list. This function is intended only for internal use by
* the IOB module.
*
****************************************************************************/
FAR struct iob_qentry_s *iob_alloc_qentry(void);
/****************************************************************************
* Name: iob_tryalloc_qentry
*
* Description:
* Try to allocate an I/O buffer chain container by taking the buffer at
* the head of the free list without waiting for the container to become
* free. This function is intended only for internal use by the IOB module.
*
****************************************************************************/
FAR struct iob_qentry_s *iob_tryalloc_qentry(void);
/****************************************************************************
* Name: iob_free_qentry
*
* Description:
* Free the I/O buffer chain container by returning it to the free list.
* The link to the next I/O buffer in the chain is return.
*
****************************************************************************/
FAR struct iob_qentry_s *iob_free_qentry(FAR struct iob_qentry_s *iobq);
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/****************************************************************************
* Name: iob_notifier_signal
*
* Description:
* An IOB has become available. Signal all threads waiting for an IOB
* that an IOB is available.
*
* When an IOB becomes available, *all* of the waiters in this thread will
* be signaled. If there are multiple waiters then only the highest
* priority thread will get the IOB. Lower priority threads will need to
* call iob_notifier_setup() once again.
*
* Input Parameters:
* None.
*
* Returned Value:
* None.
*
****************************************************************************/
#ifdef CONFIG_IOB_NOTIFIER
void iob_notifier_signal(void);
#endif
#endif /* CONFIG_MM_IOB */
#endif /* __MM_IOB_IOB_H */