From bb207a1d42c9a75a64af35de4f12912176ebfc8d Mon Sep 17 00:00:00 2001
From: rsc <rsc>
Date: Thu, 7 Sep 2006 14:28:12 +0000
Subject: [PATCH] comments
---
bio.c | 41 ++++++++++++++++++++++++++++++++----
file.c | 11 +++++-----
fs.c | 66 ++++++++++++++++++++++++++++++++++++++++++++++++++++------
ide.c | 7 +++++++
4 files changed, 109 insertions(+), 16 deletions(-)
diff --git a/bio.c b/bio.c
index ab5f41f..dadd1f7 100644
--- a/bio.c
+++ b/bio.c
@@ -1,3 +1,29 @@
+// Buffer cache.
+//
+// The buffer cache is a linked list of buf structures
+// holding cached copies of disk block contents.
+// Each buf has two state bits B_BUSY and B_VALID.
+// If B_BUSY is set, it means that some code is currently
+// editing buf, so other code is not allowed to look at it.
+// To wait for a buffer that is B_BUSY, sleep on buf.
+// (See bget below.)
+//
+// If B_VALID is set, it means that the memory contents
+// have been initialized by reading them off the disk.
+// (Conversely, if B_VALID is not set, the memory contents
+// of buf must be initialized, often by calling bread,
+// before being used.)
+//
+// After making changes to a buf's memory, call bwrite to flush
+// the changes out to disk, to keep the disk and memory copies
+// in sync.
+//
+// When finished with a buffer, call brelse to release the buffer
+// (i.e., clear B_BUSY), so that others can access it.
+//
+// Bufs that are not B_BUSY are fair game for reuse for other
+// disk blocks. It is not allowed to use a buf after calling brelse.
+
#include "types.h"
#include "param.h"
#include "x86.h"
@@ -10,7 +36,7 @@
struct buf buf[NBUF];
struct spinlock buf_table_lock;
-// linked list of all buffers, through prev/next.
+// Linked list of all buffers, through prev/next.
// bufhead->next is most recently used.
// bufhead->tail is least recently used.
struct buf bufhead;
@@ -22,6 +48,7 @@ binit(void)
initlock(&buf_table_lock, "buf_table");
+ // Create linked list of buffers
bufhead.prev = &bufhead;
bufhead.next = &bufhead;
for(b = buf; b < buf+NBUF; b++){
@@ -32,7 +59,10 @@ binit(void)
}
}
-struct buf*
+// Look through buffer cache for block n on device dev.
+// If not found, allocate fresh block.
+// In either case, return locked buffer.
+static struct buf*
getblk(uint dev, uint sector)
{
struct buf *b;
@@ -63,11 +93,12 @@ getblk(uint dev, uint sector)
return b;
}
}
- panic("getblk: no buffers");
+ panic("bget: no buffers");
}
}
}
+// Read buf's contents from disk.
struct buf*
bread(uint dev, uint sector)
{
@@ -75,7 +106,7 @@ bread(uint dev, uint sector)
struct buf *b;
extern struct spinlock ide_lock;
- b = getblk(dev, sector);
+ b = bget(dev, sector);
if(b->flags & B_VALID)
return b;
@@ -89,6 +120,7 @@ bread(uint dev, uint sector)
return b;
}
+// Write buf's contents to disk.
void
bwrite(struct buf *b, uint sector)
{
@@ -103,6 +135,7 @@ bwrite(struct buf *b, uint sector)
release(&ide_lock);
}
+// Release the buffer buf.
void
brelse(struct buf *b)
{
diff --git a/file.c b/file.c
index 4c314ec..95a249b 100644
--- a/file.c
+++ b/file.c
@@ -41,8 +41,7 @@ filealloc(void)
return 0;
}
-// Write to file descriptor;
-// addr is a kernel address, pointing into some process's p->mem.
+// Write to file f. Addr is kernel address.
int
filewrite(struct file *fd, char *addr, int n)
{
@@ -64,7 +63,7 @@ filewrite(struct file *fd, char *addr, int n)
}
}
-// Read from file descriptor.
+// Read from file f. Addr is kernel address.
int
fileread(struct file *fd, char *addr, int n)
{
@@ -85,7 +84,7 @@ fileread(struct file *fd, char *addr, int n)
}
}
-// Close file descriptor.
+// Close file f. (Decrement ref count, close when reaches 0.)
void
fileclose(struct file *fd)
{
@@ -113,7 +112,7 @@ fileclose(struct file *fd)
}
}
-// Get metadata about file descriptor.
+// Get metadata about file f.
int
filestat(struct file *fd, struct stat *st)
{
@@ -126,7 +125,7 @@ filestat(struct file *fd, struct stat *st)
return -1;
}
-// Increment file descriptor reference count.
+// Increment ref count for file f.
void
fileincref(struct file *fd)
{
diff --git a/fs.c b/fs.c
index 317a9e2..341a586 100644
--- a/fs.c
+++ b/fs.c
@@ -11,8 +11,24 @@
#include "fsvar.h"
#include "dev.h"
-// these are inodes currently in use
-// an entry is free if count == 0
+// Inode table. The inode table is an in-memory cache of the
+// on-disk inode structures. If an inode in the table has a non-zero
+// reference count, then some open files refer to it and it must stay
+// in memory. If an inode has a zero reference count, it is only in
+// memory as a cache in hopes of being used again (avoiding a disk read).
+// Any inode with reference count zero can be evicted from the table.
+//
+// In addition to having a reference count, inodes can be marked busy
+// (just like bufs), meaning that some code has logically locked the
+// inode, and others are not allowed to look at it.
+// This locking can last for a long
+// time (for example, if the inode is busy during a disk access),
+// so we don't use spin locks. Instead, if a process wants to use
+// a particular inode, it must sleep(ip) to wait for it to be not busy.
+// See iget below.
+//
+// XXX Inodes with dev == 0 exist only in memory. They have no on-disk
+// representation. This functionality is used to implement pipes.
struct inode inode[NINODE];
struct spinlock inode_table_lock;
@@ -61,6 +77,7 @@ balloc(uint dev)
return b;
}
+// Free a disk block.
static void
bfree(int dev, uint b)
{
@@ -108,6 +125,10 @@ iget(uint dev, uint inum)
if(ip->count > 0 && ip->dev == dev && ip->inum == inum){
if(ip->busy){
sleep(ip, &inode_table_lock);
+ // Since we droped inode_table_lock, ip might have been reused
+ // for some other inode entirely. Must start the scan over,
+ // and hopefully this time we will find the inode we want
+ // and it will not be busy.
goto loop;
}
ip->count++;
@@ -142,6 +163,8 @@ iget(uint dev, uint inum)
return nip;
}
+// Copy ip->d, which has changed, to disk.
+// Caller must have locked ip.
void
iupdate(struct inode *ip)
{
@@ -160,6 +183,8 @@ iupdate(struct inode *ip)
brelse(bp);
}
+// Allocate a new inode with the given type
+// from the file system on device dev.
struct inode*
ialloc(uint dev, short type)
{
@@ -195,6 +220,7 @@ ialloc(uint dev, short type)
return ip;
}
+// Free the given inode from its file system.
static void
ifree(struct inode *ip)
{
@@ -202,6 +228,11 @@ ifree(struct inode *ip)
iupdate(ip);
}
+// Lock the given inode (wait for it to be not busy,
+// and then ip->busy).
+// Caller must already hold a reference to ip.
+// Otherwise, if all the references to ip go away,
+// it might be reused underfoot.
void
ilock(struct inode *ip)
{
@@ -217,8 +248,9 @@ ilock(struct inode *ip)
release(&inode_table_lock);
}
-// caller is holding onto a reference to this inode, but no
-// longer needs to examine or change it, so clear ip->busy.
+// Caller holds reference to ip and has locked it.
+// Caller no longer needs to examine / change it.
+// Unlock it, but keep the reference.
void
iunlock(struct inode *ip)
{
@@ -233,6 +265,7 @@ iunlock(struct inode *ip)
release(&inode_table_lock);
}
+// Return the disk block address of the nth block in inode ip.
uint
bmap(struct inode *ip, uint bn)
{
@@ -259,6 +292,7 @@ bmap(struct inode *ip, uint bn)
return x;
}
+// Truncate the inode ip, discarding all its data blocks.
void
itrunc(struct inode *ip)
{
@@ -286,8 +320,9 @@ itrunc(struct inode *ip)
iupdate(ip);
}
-// caller is releasing a reference to this inode.
-// you must have the inode lock.
+// Caller holds reference to ip and has locked it,
+// possibly editing it.
+// Release lock and drop the reference.
void
iput(struct inode *ip)
{
@@ -308,6 +343,8 @@ iput(struct inode *ip)
release(&inode_table_lock);
}
+// Caller holds reference to ip but not lock.
+// Drop reference.
void
idecref(struct inode *ip)
{
@@ -315,6 +352,7 @@ idecref(struct inode *ip)
iput(ip);
}
+// Increment reference count for ip.
void
iincref(struct inode *ip)
{
@@ -323,6 +361,8 @@ iincref(struct inode *ip)
iunlock(ip);
}
+// Copy stat information from inode.
+// XXX Assumes inode is from disk file system.
void
stati(struct inode *ip, struct stat *st)
{
@@ -335,6 +375,8 @@ stati(struct inode *ip, struct stat *st)
#define min(a, b) ((a) < (b) ? (a) : (b))
+// Read data from inode.
+// XXX Assumes inode is from disk file system.
int
readi(struct inode *ip, char *dst, uint off, uint n)
{
@@ -361,6 +403,7 @@ readi(struct inode *ip, char *dst, uint off, uint n)
return target - n;
}
+// Allocate the nth block in inode ip if necessary.
static int
newblock(struct inode *ip, uint lbn)
{
@@ -396,6 +439,8 @@ newblock(struct inode *ip, uint lbn)
return 0;
}
+// Write data to inode.
+// XXX Assumes inode is from disk file system.
int
writei(struct inode *ip, char *addr, uint off, uint n)
{
@@ -551,6 +596,8 @@ namei(char *path, int mode, uint *ret_off,
}
}
+// Write a new directory entry (name, ino) into the directory dp.
+// Caller must have locked dp.
void
wdir(struct inode *dp, char *name, uint ino)
{
@@ -575,6 +622,8 @@ wdir(struct inode *dp, char *name, uint ino)
panic("wdir write");
}
+// Create the path cp and return its locked inode structure.
+// If cp already exists, return 0.
struct inode*
mknod(char *cp, short type, short major, short minor)
{
@@ -591,6 +640,9 @@ mknod(char *cp, short type, short major, short minor)
return ip;
}
+// Create a new inode named name inside dp
+// and return its locked inode structure.
+// If name already exists, return 0.
struct inode*
mknod1(struct inode *dp, char *name, short type, short major, short minor)
{
@@ -611,6 +663,7 @@ mknod1(struct inode *dp, char *name, short type, short major, short minor)
return ip;
}
+// Unlink the inode named cp.
int
unlink(char *cp)
{
@@ -649,6 +702,7 @@ unlink(char *cp)
return 0;
}
+// Create the path new as a link to the same inode as old.
int
link(char *name1, char *name2)
{
diff --git a/ide.c b/ide.c
index 4e2e1a6..3afd19b 100644
--- a/ide.c
+++ b/ide.c
@@ -17,6 +17,12 @@
#define IDE_CMD_READ 0x20
#define IDE_CMD_WRITE 0x30
+// IDE request queue.
+// The next request will be stored in request[head],
+// and the request currently being served by the disk
+// is request[tail].
+// Must hold ide_lock while manipulating queue.
+
struct ide_request {
int diskno;
uint secno;
@@ -28,6 +34,7 @@ struct ide_request {
struct ide_request request[NREQUEST];
int head, tail;
struct spinlock ide_lock;
+
int disk_1_present;
int disk_channel;
--
GitLab