关于 csapp 的一道 生产者－消费者 练习题的困惑

[mstar]

最近在读 csapp ，第 12 章的练习题 12.9 ， A 场景，答案说是生产者和消费者会并发访问缓冲区，所以必须要有互斥锁。我没有太理解，我觉得虽然它们会并发访问同一个 buf ，但并不会并发访问同一个 item ，因为生产者和消费者最接近的时候也隔着一个 item ，所以我觉得这种场景下互斥锁不是必须的。大家能帮我下？（第一次发帖，如果有不对的请轻拍:)）

以下是sbuf_t

typedef struct {
	int *buf; /* Buffer array */
	int n; /* Maximum number of slots */
	int front; /* buf[(front+1)%n] is first item */
	int rear; /* buf[rear%n] is last item */
	sem_t mutex; /* Protects accesses to buf */
	sem_t slots; /* Counts available slots */
	sem_t items; /* Counts available items */
} sbuf_t;

以下是SBUF函数实现

#include "csapp.h"
#include "sbuf.h"

/* Create an empty, bounded, shared FIFO buffer with n slots */
void sbuf_init(sbuf_t *sp, int n)
{
	sp->buf = Calloc(n, sizeof(int));
	sp->n = n; /* Buffer holds max of n items */
	sp->front = sp->rear = 0; /* Empty buffer iff front == rear */
	Sem_init(&sp->mutex, 0, 1); /* Binary semaphore for locking */
	Sem_init(&sp->slots, 0, n); /* Initially, buf has n empty slots */
	Sem_init(&sp->items, 0, 0); /* Initially, buf has zero data items */
}

/* Clean up buffer sp */
void sbuf_deinit(sbuf_t *sp)
{
	Free(sp->buf);
}

/* Insert item onto the rear of shared buffer sp */
void sbuf_insert(sbuf_t *sp, int item)
{
	P(&sp->slots); /* Wait for available slot */
	P(&sp->mutex); /* Lock the buffer */
	sp->buf[(++sp->rear)%(sp->n)] = item; /* Insert the item */
	V(&sp->mutex); /* Unlock the buffer */
	V(&sp->items); /* Announce available item */
}

/* Remove and return the first item from buffer sp */
int sbuf_remove(sbuf_t *sp)
{
	int item;
	P(&sp->items); /* Wait for available item */
	P(&sp->mutex); /* Lock the buffer */
	item = sp->buf[(++sp->front)%(sp->n)]; /* Remove the item */
	V(&sp->mutex); /* Unlock the buffer */
	V(&sp->slots); /* Announce available slot */
	return item;
}

sbuf_init()函数初始化一次，然后生产者调用sbuf_insert()，消费者调用sbuf_remove()。

以下是题目

Let p denote the number of producers, c the number of consumers, and n the
buffer size in units of items. For each of the following scenarios, indicate whether
the mutex semaphore in sbuf_insert and sbuf_remove is necessary or not.
A. p = 1, c = 1, n > 1
B. p = 1, c = 1, n = 1
C. p > 1, c > 1, n = 1

[iEverX]

同样的疑问，但是网上搜不到。同认为 A 可以不要 mutex