ll_rw_blk.c

一份精简的linux内核源代码

/*
 *  linux/kernel/blk_dev/ll_rw.c
 *
 * (C) 1991 Linus Torvalds
 */

/*
 * This handles all read/write requests to block devices
 */
#include <errno.h>
#include <linux/sched.h>
#include <linux/kernel.h>
#include <asm/system.h>

#include "blk.h"

/*
 * The request-struct contains all necessary data
 * to load a nr of sectors into memory
 */
struct request request[NR_REQUEST];

/*
 * used to wait on when there are no free requests
 */
struct task_struct * wait_for_request = NULL;

/* blk_dev_struct is:
 *	do_request-address
 *	next-request
 */
struct blk_dev_struct blk_dev[NR_BLK_DEV] = {
	{ NULL, NULL },		/* no_dev */
	{ NULL, NULL },		/* dev mem */
	{ NULL, NULL },		/* dev fd */
	{ NULL, NULL },		/* dev hd */
	{ NULL, NULL },		/* dev ttyx */
	{ NULL, NULL },		/* dev tty */
	{ NULL, NULL }		/* dev lp */
};

static inline void lock_buffer(struct buffer_head * bh)/*锁定指定缓冲块，若该块已被锁定，则当前进程自己睡眠（不可中断等待）*/
{
	cli(); /* 对于缓冲块等共享资源在分配时需关中断 */
	while (bh->b_lock)   /*被唤醒的进程从这开始运行，继续申请缓冲区*/
		sleep_on(&bh->b_wait);
	bh->b_lock=1;
	sti();
}

static inline void unlock_buffer(struct buffer_head * bh)
{
	if (!bh->b_lock)
		printk("ll_rw_block.c: buffer not locked\n\r");
	bh->b_lock = 0;
	wake_up(&bh->b_wait);
}

/*
 * add-request adds a request to the linked list.
 * It disables interrupts so that it can muck with the
 * request-lists in peace.
 */
static void add_request(struct blk_dev_struct * dev, struct request * req) /* 将请求项加入相应块设备的请求队列，若队列为空则直接调用块设备的处理函数 */
{
	struct request * tmp;

	req->next = NULL;/* 初始化请求项：next置为空；清缓冲区的脏标志 */
	cli(); /*对缓冲区这类共享资源在使用时需关中断 */
	if (req->bh)
		req->bh->b_dirt = 0;
	if (!(tmp = dev->current_request)) {/* 查看当前设备是否有请求项，若没有则直接调用块设备的处理程序 */
		dev->current_request = req;
		sti();
		(dev->request_fn)();
		return;
	}
	for ( ; tmp->next ; tmp=tmp->next)/* 若请求项队列非空，则将新请求项插入队列中（电梯算法） */
		if ((IN_ORDER(tmp,req) ||
		    !IN_ORDER(tmp,tmp->next)) &&
		    IN_ORDER(req,tmp->next))
			break;
	req->next=tmp->next;
	tmp->next=req;
	sti();
}

static void make_request(int major,int rw, struct buffer_head * bh)
{
	struct request * req;
	int rw_ahead;

/* WRITEA/READA is special case - it is not really needed, so if the */
/* buffer is locked, we just forget about it, else it's a normal read */
	if (rw_ahead = (rw == READA || rw == WRITEA)) { /* 预读写命令（其实就是将预读写命令该为读写命令） */
		if (bh->b_lock)
			return;
		if (rw == READA)
			rw = READ;
		else
			rw = WRITE;
	}
	if (rw!=READ && rw!=WRITE)
		panic("Bad block dev command, must be R/W/RA/WA");
	lock_buffer(bh);/*缓冲区为共享资源，故在使用前需上锁*/
	if ((rw == WRITE && !bh->b_dirt) || (rw == READ && bh->b_uptodate)) {/* 这里体现了缓冲区的用意（b_dirt标志表示缓冲块中的数据是否已被修改过。b_update标志用于表示缓冲块中的数据是与块设备上同步的） */
		unlock_buffer(bh);
		return;
	}
repeat:
/* we don't allow the write-requests to fill up the queue completely:
 * we want some room for reads: they take precedence. The last third
 * of the requests are only for reads.
 */
	if (rw == READ)/* 请求项数组的后1/3为读请求项 */
		req = request+NR_REQUEST;
	else
		req = request+((NR_REQUEST*2)/3);
/* find an empty request */
	while (--req >= request)
		if (req->dev<0)
			break;
/* if none found, sleep on new requests: check for rw_ahead */
	if (req < request) {
		if (rw_ahead) {
			unlock_buffer(bh);
			return;
		}
		sleep_on(&wait_for_request);/* 等待空请求项而被挂起的进程将从这开始继续运行*/
		goto repeat;
	}
/* fill up the request-info, and add it to the queue */
	req->dev = bh->b_dev;
	req->cmd = rw;
	req->errors=0;
	req->sector = bh->b_blocknr<<1;
	req->nr_sectors = 2;
	req->buffer = bh->b_data;
	req->waiting = NULL;
	req->bh = bh;
	req->next = NULL;
	add_request(major+blk_dev,req);
}

void ll_rw_block(int rw, struct buffer_head * bh)
{
	unsigned int major;

	if ((major=MAJOR(bh->b_dev)) >= NR_BLK_DEV ||
	!(blk_dev[major].request_fn)) {
		printk("Trying to read nonexistent block-device\n\r");
		return;
	}
	make_request(major,rw,bh);
}

void blk_dev_init(void)/*块设备初始化*/
{
	int i;

	for (i=0 ; i<NR_REQUEST ; i++) {/*初始化块设备请求数组，将所有请求设置为空闲*/
		request[i].dev = -1;
		request[i].next = NULL;
	}
}