Commit graph

106 commits

Author SHA1 Message Date
rsc
f97f0d2b3d cleaner 2007-09-27 21:02:03 +00:00
rsc
3807c1f20b rename splhi/spllo to pushcli/popcli 2007-09-27 20:09:40 +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
rsc
d5596cd61d Apparently the initial interrupt count lapic[TICR]
must be set *after* initializing the lapic[TIMER] vector.

Doing this, we now get clock interrupts on cpu 1.
(No idea why we always got them on cpu 0.)

Don't write to TCCR - it is read-only.
2007-09-26 20:34:12 +00:00
rsc
cffa954301 nits 2007-08-28 19:25:04 +00:00
rsc
5573c8f296 delete proc_ on proc_exit, proc_wait, proc_kill 2007-08-28 19:14:43 +00:00
rsc
eb52c7de1d comments; rename irq_ to pic_ 2007-08-28 19:04:36 +00:00
rsc
818fc0125e replace setjmp/longjmp with swtch 2007-08-28 12:48:33 +00:00
rsc
9e82bfb04c rename 8253pit.c to timer.c 2007-08-28 04:40:58 +00:00
rsc
7834cca604 remove _ from pipe; be like file 2007-08-28 04:22:35 +00:00
rsc
f0d11fea82 Move keyboard code into kbd.c; add backspace handling. 2007-08-28 03:28:13 +00:00
rsc
19b42cc078 Rename main0 to main. 2007-08-27 23:32:16 +00:00
rsc
b63bb0fd00 Clean up lapic code.
One initialization function now, not three.
Use #defines instead of enums (consistent with other code, but sigh).

Still boots in Bochs in SMP mode.
2007-08-27 16:57:13 +00:00
rsc
1ccff18b24 fileincref -> filedup (consistent with idup) 2007-08-27 14:35:09 +00:00
rsc
efc12b8e61 Replace yield system call with sleep. 2007-08-27 13:34:35 +00:00
rsc
7bb73cdbc7 nits 2007-08-27 12:48:20 +00:00
rsc
766ba5cc06 first ever correct use of strncpy 2007-08-24 21:00:02 +00:00
rsc
07090dd705 Remove struct uinode.
Remove type arg to mknod (assume T_DEV).
2007-08-24 20:54:23 +00:00
rtm
902b13f5d6 simplify ide queuing
nits in comments
2007-08-24 19:32:36 +00:00
rsc
649bc99fe4 align, sort 2007-08-23 23:58:19 +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
fbf9103952 remove namelen parameter 2007-08-21 19:58:55 +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
bcca6c6bde shuffle fs.c in bottom-up order 2007-08-20 18:23:52 +00:00
rsc
c65e499f3b help gcc 2007-08-08 10:27:04 +00:00
rsc
9ad44da676 unused 2007-08-08 09:44:23 +00:00
rsc
453c6a65a2 convert syscall dispatch to table 2007-08-08 09:41:21 +00:00
rsc
d80b06a1e0 iincref returns new ref 2007-08-08 09:30:42 +00:00
rsc
7e89fb90bd add safestrcpy 2007-08-08 08:37:22 +00:00
rsc
3689c7f5ce unnecessary struct jmpbuf 2006-09-08 15:31:18 +00:00
rsc
66d46d691d fix file names 2006-09-08 14:40:51 +00:00
rsc
41abfaa92e missing def 2006-09-07 16:54:23 +00:00
rsc
ab17e3198b debugging prints 2006-09-07 15:45:38 +00:00
rsc
19297caf0d fix ide, pit interfaces 2006-09-07 15:29:54 +00:00
rsc
224f6598c1 refactor syscall code 2006-09-07 14:13:26 +00:00
kaashoek
f70172129c run without lapic and ioapic, if they are not present
if no lapic available, use 8253pit for clock
now xv6 runs both on qemu (uniprocessor) and bochs (uniprocessor and MP)
2006-09-07 01:37:58 +00:00
rsc
d6b20251f0 fd.c -> file.c 2006-09-06 19:47:07 +00:00
rsc
50e514be98 fd_* => file_* 2006-09-06 18:43:45 +00:00
rsc
39593d2f1a struct fd -> struct file 2006-09-06 18:38:56 +00:00
rsc
2685309fb4 split syscall.c into sysfile.c and sysproc.c 2006-09-06 18:19:11 +00:00
rsc
9e9bcaf143 standardize various * conventions 2006-09-06 17:27:19 +00:00
rsc
03b6376f56 standardize on unnamed args in prototypes 2006-09-06 17:10:00 +00:00
rtm
2aa4c3bc29 complain if no disk 1
lots of cleanup
2006-08-30 18:55:06 +00:00
rtm
3b95801add i broke sbrk, fix it 2006-08-29 17:01:40 +00:00
rtm
2b19190c13 clean up stale error checks and panics
delete unused functions
a few comments
2006-08-29 14:45:45 +00:00
rtm
5051da6de3 inode addrs[NDIRECT] -> NADDRS
fix race in mknod / creat
use last component in dirent in mknod, not path
2006-08-25 01:11:30 +00:00
kaashoek
ea2909b6b5 user-level malloc (untested)
nit in sbrk
indirect block
fix dup to share fd struct
2006-08-24 02:44:41 +00:00
kaashoek
8b58e81077 i/o redirection in sh
better parsing of sh commands (copied from jos sh)
cat: read from 1 if no args
sbrk system call, but untested
getpid system call
moved locks in keyboard intr, but why do we get intr w. null characters from keyboard?
2006-08-23 01:09:24 +00:00
kaashoek
f18ab5c04e compiling, but untested dup 2006-08-20 03:33:01 +00:00