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94 commits

Author SHA1 Message Date
rtm fd6b029401 proc_wait -> wait 2007-10-20 18:25:38 +00:00
rsc ab08960f64 Final word on the locking fiasco?
Change pushcli / popcli so that they can never turn on
interrupts unexpectedly.  That is, if interrupts are on,
then pushcli(); popcli(); turns them off and back on, but
if they are off to begin with, then pushcli(); popcli(); is
a no-op.

I think our fundamental mistake was having a primitive
(release and then popcli nee spllo) that could turn
interrupts on at unexpected moments instead of being
explicit about when we want to start allowing interrupts.

With the new semantics, all the manual fiddling of ncli
to force interrupts off in certain sections goes away.
In return, we must explicitly mark the places where
we want to enable interrupts unconditionally, by calling sti().
There is only one: inside the scheduler loop.
2007-09-27 21:25:37 +00:00
rsc c95bde8163 yank out stack overflow checking ugliness 2007-09-27 20:38:53 +00:00
rsc 4f74de0edc okay, that was long enough - revert 2007-09-27 20:32:45 +00:00
rsc ce2e751555 test: store curproc at top of stack
I don't actually think this is worthwhile, but I figured
I would check it in before reverting it, so that it can
be in the revision history.

Pros:
  * curproc doesn't need to turn on/off interrupts
  * scheduler doesn't have to edit curproc anymore

Cons:
  * it's ugly
  * all the stack computation is more complicated.
  * it doesn't actually simplify anything but curproc,
    and even curproc is harder to follow.
2007-09-27 20:29:50 +00:00
rsc 3807c1f20b rename splhi/spllo to pushcli/popcli 2007-09-27 20:09:40 +00:00
rsc 39c3fb1b15 overkill: use segments to catch stack overflow (delete before next year) 2007-09-27 19:39:10 +00:00
rsc c8919e6537 kernel SMP interruptibility fixes.
Last year, right before I sent xv6 to the printer, I changed the
SETGATE calls so that interrupts would be disabled on entry to
interrupt handlers, and I added the nlock++ / nlock-- in trap()
so that interrupts would stay disabled while the hw handlers
(but not the syscall handler) did their work.  I did this because
the kernel was otherwise causing Bochs to triple-fault in SMP
mode, and time was short.

Robert observed yesterday that something was keeping the SMP
preemption user test from working.  It turned out that when I
simplified the lapic code I swapped the order of two register
writes that I didn't realize were order dependent.  I fixed that
and then since I had everything paged in kept going and tried
to figure out why you can't leave interrupts on during interrupt
handlers.  There are a few issues.

First, there must be some way to keep interrupts from "stacking
up" and overflowing the stack.  Keeping interrupts off the whole
time solves this problem -- even if the clock tick handler runs
long enough that the next clock tick is waiting when it finishes,
keeping interrupts off means that the handler runs all the way
through the "iret" before the next handler begins.  This is not
really a problem unless you are putting too many prints in trap
-- if the OS is doing its job right, the handlers should run
quickly and not stack up.

Second, if xv6 had page faults, then it would be important to
keep interrupts disabled between the start of the interrupt and
the time that cr2 was read, to avoid a scenario like:

   p1 page faults [cr2 set to faulting address]
   p1 starts executing trapasm.S
   clock interrupt, p1 preempted, p2 starts executing
   p2 page faults [cr2 set to another faulting address]
   p2 starts, finishes fault handler
   p1 rescheduled, reads cr2, sees wrong fault address

Alternately p1 could be rescheduled on the other cpu, in which
case it would still see the wrong cr2.  That said, I think cr2
is the only interrupt state that isn't pushed onto the interrupt
stack atomically at fault time, and xv6 doesn't care.  (This isn't
entirely hypothetical -- I debugged this problem on Plan 9.)

Third, and this is the big one, it is not safe to call cpu()
unless interrupts are disabled.  If interrupts are enabled then
there is no guarantee that, between the time cpu() looks up the
cpu id and the time that it the result gets used, the process
has not been rescheduled to the other cpu.  For example, the
very commonly-used expression curproc[cpu()] (aka the macro cp)
can end up referring to the wrong proc: the code stores the
result of cpu() in %eax, gets rescheduled to the other cpu at
just the wrong instant, and then reads curproc[%eax].

We use curproc[cpu()] to get the current process a LOT.  In that
particular case, if we arranged for the current curproc entry
to be addressed by %fs:0 and just use a different %fs on each
CPU, then we could safely get at curproc even with interrupts
disabled, since the read of %fs would be atomic with the read
of %fs:0.  Alternately, we could have a curproc() function that
disables interrupts while computing curproc[cpu()].  I've done
that last one.

Even in the current kernel, with interrupts off on entry to trap,
interrupts are enabled inside release if there are no locks held.
Also, the scheduler's idle loop must be interruptible at times
so that the clock and disk interrupts (which might make processes
runnable) can be handled.

In addition to the rampant use of curproc[cpu()], this little
snippet from acquire is wrong on smp:

  if(cpus[cpu()].nlock == 0)
    cli();
  cpus[cpu()].nlock++;

because if interrupts are off then we might call cpu(), get
rescheduled to a different cpu, look at cpus[oldcpu].nlock, and
wrongly decide not to disable interrupts on the new cpu.  The
fix is to always call cli().  But this is wrong too:

  if(holding(lock))
    panic("acquire");
  cli();
  cpus[cpu()].nlock++;

because holding looks at cpu().  The fix is:

  cli();
  if(holding(lock))
    panic("acquire");
  cpus[cpu()].nlock++;

I've done that, and I changed cpu() to complain the first time
it gets called with interrupts disabled.  (It gets called too
much to complain every time.)

I added new functions splhi and spllo that are like acquire and
release but without the locking:

  void
  splhi(void)
  {
    cli();
    cpus[cpu()].nsplhi++;
  }

  void
  spllo(void)
  {
    if(--cpus[cpu()].nsplhi == 0)
      sti();
  }

and I've used those to protect other sections of code that refer
to cpu() when interrupts would otherwise be disabled (basically
just curproc and setupsegs).  I also use them in acquire/release
and got rid of nlock.

I'm not thrilled with the names, but I think the concept -- a
counted cli/sti -- is sound.  Having them also replaces the
nlock++/nlock-- in trap.c and main.c, which is nice.


Final note: it's still not safe to enable interrupts in
the middle of trap() between lapic_eoi and returning
to user space.  I don't understand why, but we get a
fault on pop %es because 0x10 is a bad segment
descriptor (!) and then the fault faults trying to go into
a new interrupt because 0x8 is a bad segment descriptor too!
Triple fault.  I haven't debugged this yet.
2007-09-27 12:58:42 +00:00
rtm de1329dda2 longjmp -> swtch in comments 2007-08-30 17:39:56 +00:00
rsc 5573c8f296 delete proc_ on proc_exit, proc_wait, proc_kill 2007-08-28 19:14:43 +00:00
rsc 5516be1fed spaces around else for rtm 2007-08-28 18:37:41 +00:00
rsc e4d6a21165 more consistent spacing 2007-08-28 18:32:08 +00:00
rsc fc21046754 nit 2007-08-28 12:52:14 +00:00
rsc 818fc0125e replace setjmp/longjmp with swtch 2007-08-28 12:48:33 +00:00
rsc 558ab49f13 delete unnecessary #include lines 2007-08-27 23:26:33 +00:00
rsc 1ccff18b24 fileincref -> filedup (consistent with idup) 2007-08-27 14:35:09 +00:00
rsc 97ac612fb1 nits 2007-08-24 20:28:08 +00:00
rsc dd86897434 make wakeup1 static 2007-08-24 20:22:55 +00:00
rsc b1fb19b6df Use parent pointer instead of ppid. 2007-08-23 14:40:30 +00:00
rsc 3a057d12ae avoid hardcoding init as pid 1 proc[0] 2007-08-23 14:35:28 +00:00
rsc 1d7839a1da my mistake found by robert 2007-08-22 17:45:52 +00:00
rsc eaea18cb9c 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
2007-08-22 06:01:32 +00:00
rsc f32f3638f4 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].
2007-08-21 19:22:08 +00:00
rsc e2a620da49 checkpoint - simpler namei interface 2007-08-20 19:37:15 +00:00
rsc 07ddc0fa82 nit 2007-08-14 19:41:29 +00:00
rsc 2ef3a64bb4 Because limit gives the address of the last addressable
byte in the segment, the limit argument to SEG16 and SEG
needs to have 1 subtracted from it.
2007-08-14 04:55:45 +00:00
rsc b6095304b7 Make cp a magic symbol. 2007-08-10 16:37:27 +00:00
rsc 9583b476bf try to use cp only for curproc[cpu()] 2007-08-09 17:32:40 +00:00
rsc f2f062da61 check p->killed for long-lived sleeps 2007-08-08 10:29:42 +00:00
rsc c16e0916a7 cleaner table 2007-08-08 09:43:07 +00:00
rsc 19b1f63813 cleaner table 2007-08-08 09:42:36 +00:00
rsc d80b06a1e0 iincref returns new ref 2007-08-08 09:30:42 +00:00
rsc b6dc6187f7 add DPL_USER constant 2007-08-08 09:02:42 +00:00
rsc 00d0f794cf fix various bugs 2007-08-08 08:57:37 +00:00
rsc a7d18bb6f0 comment tweaks; more informative process list 2007-08-08 08:38:38 +00:00
rsc 1656b1b232 move growproc up higher 2006-09-08 14:26:51 +00:00
rsc d911d83ca1 fix various comments 2006-09-08 13:55:43 +00:00
rsc b6cac0a53b check preconditions in sched 2006-09-07 16:54:00 +00:00
rsc ab17e3198b debugging prints 2006-09-07 15:45:38 +00:00
rsc 31085bb416 more comments 2006-09-07 14:12:30 +00:00
rsc 1133b215d8 more defensive programming 2006-09-07 01:56:22 +00:00
rsc 50e514be98 fd_* => file_* 2006-09-06 18:43:45 +00:00
rsc 9936bffa45 fd.* -> file.* 2006-09-06 18:40:28 +00:00
rsc 39593d2f1a struct fd -> struct file 2006-09-06 18:38:56 +00:00
rsc f552738889 no /* */ comments 2006-09-06 17:50:20 +00:00
rsc 9e9bcaf143 standardize various * conventions 2006-09-06 17:27:19 +00:00
rsc 0992df4b90 clear np->mem after freeing 2006-09-06 16:35:21 +00:00
kaashoek 21a88dd053 some pipe support in sh
bug in proc_wait
2006-09-06 15:32:21 +00:00
rtm 2aa4c3bc29 complain if no disk 1
lots of cleanup
2006-08-30 18:55:06 +00:00
rtm 18432ed5ed nits 2006-08-29 21:35:30 +00:00