group locks into structs they protect.
few naming nits.

Suppose an inode has been used and freed.
It is left marked I_VALID (the bug).
Now ialloc comes along and reuses the
inode.  It writes the new inode type to disk
and returns iget(dev, inum) to get the
cache entry.  Iget sees that the inode is valid
and doesn't bother refreshing from disk.
Now when the caller iupdates, it will write
out a zero type and the file or directory has
disappeared.

Change dev read/write functions
to take inode* instead of minor number.

Unlock console inode during console_read
and console_write.  Otherwise background
processes cannot write to console while the
shell is reading it waiting for input.

No one is going to find this inode.
There is only one ref to it -- the caller --
and no links to it.

FS cleanup.

Add utility routines bzero, readsb so that balloc, bfree fit on one page.
Make balloc loop clearer.

Remove struct uinode.
Remove type arg to mknod (assume T_DEV).

nits in comments

PDF at http://am.lcs.mit.edu/~rsc/xv6.pdf

Various changes made while offline.

 + bwrite sector argument is redundant; use b->sector.
 + reformatting of files for nicer PDF page breaks
 + distinguish between locked, unlocked inodes in type signatures
 + change FD_FILE to FD_INODE
 + move userinit (nee proc0init) to proc.c
 + move ROOTDEV to param.h
 + always parenthesize sizeof argument

Various cleanup:

 - Got rid of dummy proc[0].  Now proc[0] is init.
 - Added initcode.S to exec /init, so that /init is
   just a regular binary.
 - Moved exec out of sysfile to exec.c
 - Moved code dealing with fs guts (like struct inode)
   from sysfile.c to fs.c.  Code dealing with system call
   arguments stays in sysfile.c
 - Refactored directory routines in fs.c; should be simpler.
 - Changed iget to return *unlocked* inode structure.
   This solves the lookup-then-use race in namei
   without introducing deadlocks.
   It also enabled getting rid of the dummy proc[0].