xv6-cs450/Notes

bochs 2.2.6:
./configure --enable-smp --enable-disasm --enable-debugger --enable-all-optimizations --enable-4meg-pages --enable-global-pages --enable-pae --disable-reset-on-triple-fault
bochs CVS after 2.2.6:
./configure --enable-smp --enable-disasm --enable-debugger --enable-all-optimizations --enable-4meg-pages --enable-global-pages --enable-pae 

bootmain.c doesn't work right if the ELF sections aren't
sector-aligned. so you can't use ld -N. and the sections may also need
to be non-zero length, only really matters for tiny "kernels".

kernel loaded at 1 megabyte. stack same place that bootasm.S left it.

kinit() should find real mem size
  and rescue useable memory below 1 meg

no paging, no use of page table hardware, just segments

no user area: no magic kernel stack mapping
  so no copying of kernel stack during fork
  though there is a kernel stack page for each process

no kernel malloc(), just kalloc() for user core

user pointers aren't valid in the kernel

setting up first process
  we do want a process zero, as template
    but not runnable
  just set up return-from-trap frame on new kernel stack
  fake user program that calls exec

map text read-only?
shared text?

what's on the stack during a trap or sys call?
  PUSHA before scheduler switch? for callee-saved registers.
  segment contents?
  what does iret need to get out of the kernel?
  how does INT know what kernel stack to use?
 
are interrupts turned on in the kernel? probably.

per-cpu curproc
one tss per process, or one per cpu?
one segment array per cpu, or per process?

pass curproc explicitly, or implicit from cpu #?
  e.g. argument to newproc()?
  hmm, you need a global curproc[cpu] for trap() &c

test stack expansion
test running out of memory, process slots

we can't really use a separate stack segment, since stack addresses
need to work correctly as ordinary pointers. the same may be true of
data vs text. how can we have a gap between data and stack, so that
both can grow, without committing 4GB of physical memory? does this
mean we need paging?

what's the simplest way to add the paging we need?
  one page table, re-write it each time we leave the kernel?
  page table per process?
  probably need to use 0-0xffffffff segments, so that
    both data and stack pointers always work
  so is it now worth it to make a process's phys mem contiguous?
  or could use segment limits and 4 meg pages?
    but limits would prevent using stack pointers as data pointers
  how to write-protect text? not important?

perhaps have fixed-size stack, put it in the data segment?

oops, if kernel stack is in contiguous user phys mem, then moving
users' memory (e.g. to expand it) will wreck any pointers into the
kernel stack.

do we need to set fs and gs? so user processes can't abuse them?

setupsegs() may modify current segment table, is that legal?

trap() ought to lgdt on return, since currently only done in swtch()

protect hardware interrupt vectors from user INT instructions?

test out-of-fd cases for creating pipe.
test pipe reader closes then write
test two readers, two writers.
test children being inherited by grandparent &c

some sleep()s should be interruptible by kill()

cli/sti in acquire/release should nest!
  in case you acquire two locks

what would need fixing if we got rid of kernel_lock?
  console output
  proc_exit() needs lock on proc *array* to deallocate
  kill() needs lock on proc *array*
  allocator's free list
  global fd table (really free-ness)
  sys_close() on fd table
  fork on proc list, also next pid
    hold lock until public slots in proc struct initialized

locks
  init_lock
    sequences CPU startup
  proc_table_lock
    also protects next_pid
  per-fd lock *just* protects count read-modify-write
    also maybe freeness?
  memory allocator
  printf

wakeup needs proc_table_lock
  so we need recursive locks?
  or you must hold the lock to call wakeup?

in general, the table locks protect both free-ness and
  public variables of table elements
  in many cases you can use table elements w/o a lock
  e.g. if you are the process, or you are using an fd

lock code shouldn't call cprintf...

nasty hack to allow locks before first process,
  and to allow them in interrupts when curproc may be zero

race between release and sleep in sys_wait()
race between sys_exit waking up parent and setting state=ZOMBIE
race in pipe code when full/empty

lock order
  per-pipe lock
  proc_table_lock fd_table_lock kalloc_lock
  console_lock

condition variable + mutex that protects it
  proc * (for wait()), proc_table_lock
  pipe structure, pipe lock

systematic way to test sleep races?
  print something at the start of sleep?

do you have to be holding the mutex in order to call wakeup()?

device interrupts don't clear FL_IF
  so a recursive timer interrupt is possible

what does inode->busy mean?
  might be held across disk reads
  no-one is allowed to do anything to the inode
  protected by inode_table_lock
inode->count counts in-memory pointers to the struct
  prevents inode[] element from being re-used
  protected by inode_table_lock

blocks and inodes have ad-hoc sleep-locks
  provide a single mechanism?

need to lock bufs in bio between bread and brelse

test 14-character file names
and file arguments longer than 14
and directories longer than one sector

kalloc() can return 0; do callers handle this right?

why directing interrupts to cpu 1 causes trouble
  cpu 1 turns on interrupts with no tss!
    and perhaps a stale gdt (from boot)
  since it has never run a process, never called setupsegs()
  but does cpu really need the tss?
    not switching stacks
  fake process per cpu, just for tss?
    seems like a waste
  move tss to cpu[]?
    but tss points to per-process kernel stack
    would also give us a gdt
  OOPS that wasn't the problem

wait for other cpu to finish starting before enabling interrupts?
  some kind of crash in ide_init ioapic_enable cprintf
move ide_init before mp_start?
  didn't do any good
  maybe cpu0 taking ide interrupt, cpu1 getting a nested lock error

cprintfs are screwed up if locking is off
  often loops forever
  hah, just use lpt alone

looks like cpu0 took the ide interrupt and was the last to hold
the lock, but cpu1 thinks it is nested
cpu0 is in load_icode / printf / cons_putc
  probably b/c cpu1 cleared use_console_lock
cpu1 is in scheduler() / printf / acquire

  1: init timer
  0: init timer
  cpu 1 initial nlock 1
  ne0s:t iidd el_occnkt rc
  onsole cpu 1 old caller stack 1001A5 10071D 104DFF 1049FE
  panic: acquire
  ^CNext at t=33002418
  (0) [0x00100091] 0008:0x00100091 (unk. ctxt): jmp .+0xfffffffe          ; ebfe
  (1) [0x00100332] 0008:0x00100332 (unk. ctxt): jmp .+0xfffffffe          
  
why is output interleaved even before panic?

does release turn on interrupts even inside an interrupt handler?

overflowing cpu[] stack?
  probably not, change from 512 to 4096 didn't do anything


  1: init timer
  0: init timer
  cnpeus te11  linnitki aclo nnoolleek  cp1u
   ss  oarltd  sccahleldeul esrt aocnk  cpu 0111 Ej6  buf1 01A3140 C5118 
  0
  la anic1::7 0a0c0  uuirr e
  ^CNext at t=31691050
  (0) [0x00100373] 0008:0x00100373 (unk. ctxt): jmp .+0xfffffffe          ; ebfe
  (1) [0x00100091] 0008:0x00100091 (unk. ctxt): jmp .+0xfffffffe          ; ebfe

cpu0:

0: init timer
nested lock console cpu 0 old caller stack 1001e6 101a34 1 0
  (that's mpmain)
panic: acquire

cpu1:

1: init timer
cpu 1 initial nlock 1
start scheduler on cpu 1 jmpbuf ...
la 107000 lr ...
  that is, nlock != 0

maybe a race; acquire does
  locked = 1
  cpu = cpu()
what if another acquire calls holding w/ locked = 1 but
  before cpu is set?

if I type a lot (kbd), i get a panic
cpu1 in scheduler: panic "holding locks in scheduler"
cpu0 also in the same panic!
recursive interrupt?
  FL_IF is probably set during interrupt... is that correct?
again:
  olding locks in scheduler
  trap v 33 eip 100ED3 c (that is, interrupt while holding a lock)
  100ed3 is in lapic_write
again:
  trap v 33 eip 102A3C cpu 1 nlock 1 (in acquire)
  panic: interrupt while holding a lock
again:
  trap v 33 eip 102A3C cpu 1 nlock 1
  panic: interrupt while holding a lock
OR is it the cprintf("kbd overflow")?
  no, get panic even w/o that cprintf
OR a release() at interrupt time turns interrupts back on?
  of course i don't think they were off...
OK, fixing trap.c to make interrupts turn off FL_IF
  that makes it take longer, but still panics
  (maybe b/c release sets FL_IF)

shouldn't something (PIC?) prevent recursive interrupts of same IRQ?
  or should FL_IF be clear during all interrupts?

maybe acquire should remember old FL_IF value, release should restore
  if acquire did cli()

DUH the increment of nlock in acquire() happens before the cli!
  so the panic is probably not a real problem
  test nlock, cli(), then increment?

BUT now userfs doesn't do the final cat README

AND w/ cprintf("kbd overflow"), panic holding locks in scheduler
  maybe also simulataneous panic("interrupt while holding a lock")

again (holding down x key):
  kbd overflow
  kbd oaaniicloowh
  olding locks in scheduler
  trap v 33 eip 100F5F c^CNext at t=32166285
  (0) [0x0010033e] 0008:0010033e (unk. ctxt): jmp .+0xfffffffe (0x0010033e) ; ebfe
  (1) [0x0010005c] 0008:0010005c (unk. ctxt): jmp .+0xfffffffe (0x0010005c) ; ebfe
cpu0 paniced due to holding locks in scheduler
cpu1 got panic("interrupt while holding a lock")
  again in lapic_write.
  while re-enabling an IRQ?

again:
cpu 0 panic("holding locks in scheduler")
  but didn't trigger related panics earlier in scheduler or sched()
  of course the panic is right after release() and thus sti()
  so we may be seeing an interrupt that left locks held
cpu 1 unknown panic
why does it happen to both cpus at the same time?

again:
cpu 0 panic("holding locks in scheduler")
  but trap() didn't see any held locks on return
cpu 1 no apparent panic

again:
cpu 0 panic: holding too many locks in scheduler
cpu 1 panic: kbd_intr returned while holding a lock

again:
cpu 0 panic: holding too man
  la 10d70c lr 10027b
  those don't seem to be locks...
  only place non-constant lock is used is sleep()'s 2nd arg
  maybe register not preserved across context switch?
  it's in %esi...
  sched() doesn't touch %esi
  %esi is evidently callee-saved
  something to do with interrupts? since ordinarily it works
cpu 1 panic: kbd_int returned while holding a lock
  la 107340 lr 107300
  console_lock and kbd_lock

maybe console_lock is often not released due to change
  in use_console_lock (panic on other cpu)

again:
cpu 0: panic: h...
  la 10D78C lr 102CA0
cpu 1: panic: acquire FL_IF (later than cpu 0)

but if sleep() were acquiring random locks, we'd see panics
in release, after sleep() returned.
actually when system is idle, maybe no-one sleeps at all.
  just scheduler() and interrupts

questions:
  does userfs use pipes? or fork?
    no
  does anything bad happen if process 1 exits? eg exit() in cat.c
    looks ok
  are there really no processes left?
  lock_init() so we can have a magic number?

HMM maybe the variables at the end of struct cpu are being overwritten
  nlocks, lastacquire, lastrelease
  by cpu->stack?
  adding junk buffers maybe causes crash to take longer...
  when do we run on cpu stack?
  just in scheduler()?
    and interrupts from scheduler()
 
OH! recursive interrupts will use up any amount of cpu[].stack!
  underflow and wrecks *previous* cpu's struct