Commit graph

70 commits

Author SHA1 Message Date
Robert Morris 858475e4df tiny clarifications to some gdt code. 2016-08-10 10:51:14 -04:00
Robert Morris 020c8e2384 use acquire/release to force order for pid=np->pid;np->state=RUNNING
for bug reported by symingz@gmail.com and cs1100254@cse.iitd.ernet.in
2014-08-04 06:13:49 -04:00
Robert Morris 8960f60b30 junk 2012-08-28 12:57:05 -04:00
Frans Kaashoek a4b213cf49 Avoid "boot" in xv6 2011-08-15 20:11:13 -04:00
Frans Kaashoek a56c8d609b One definition of several macros and constants 2011-08-08 13:30:08 -04:00
Robert Morris faad047ab2 change some comments, maybe more informative
delete most comments from bootother.S (since copy of bootasm.S)
ksegment() -> seginit()
move more stuff from main() to mainc()
2010-09-13 15:34:44 -04:00
Austin Clements d8828817d7 Rearrange proc.h and proc.c to get our action-packed spreads back (mostly). They also make sense in this order, so it's not just for page layout. 2010-09-02 04:15:17 -04:00
Austin Clements 1280d26802 Oops, missed a comment about the stack guard 2010-09-02 03:36:39 -04:00
Robert Morris c4cc10da7e fix corner cases in exec of ELF
put an invalid page below the stack
have fork() handle invalid pages
2010-08-06 11:12:18 -04:00
Robert Morris 1afc9d3fca add some comments
find out the hard way why user and kernel must have separate segment descriptors
2010-08-05 21:16:55 -04:00
Frans Kaashoek 40889627ba Initial version of single-cpu xv6 with page tables 2010-07-02 14:51:53 -04:00
Frans Kaashoek ab777a9ad0 fine-tune comment 2009-10-07 17:42:14 -04:00
Frans Kaashoek fb4868745c fine tune comment 2009-10-07 15:31:55 -04:00
Russ Cox 7e0cc8e36e another attempt at cpu-local variables.
this time do it ourselves instead of piggybacking on TLS.
add -fno-pic to Makefile; pic code breaks our fake TLS.
2009-09-02 10:07:59 -07:00
Russ Cox f8ab2079cd fix TLS again;
still not quite but a lot better.
2009-09-02 07:59:24 -07:00
Russ Cox 57ae146362 Fix TLS for PIC systems 2009-09-02 07:41:08 -07:00
Russ Cox 48755214c9 assorted fixes:
* rename c/cp to cpu/proc
 * rename cpu.context to cpu.scheduler
 * fix some comments
 * formatting for printout
2009-08-30 23:02:08 -07:00
rsc 34295f461a group locks into structs they protect.
few naming nits.
2009-05-31 05:12:21 +00:00
rsc 030a47736f tab police 2009-05-31 00:39:17 +00:00
rsc 19333efb9e Some proc cleanup, moving some of copyproc into allocproc.
Also, an experiment: use "thread-local" storage for c and cp
instead of the #define macro for curproc[cpu()].
2009-05-31 00:28:45 +00:00
kolya deca9fef83 indent 2008-10-15 05:15:32 +00:00
kolya c100d9ee2d cleaner swtch.S 2008-10-15 05:14:10 +00:00
rsc 943fd378a1 Incorporate new understanding of/with Intel SMP spec.
Dropped cmpxchg in favor of xchg, to match lecture notes.

Use xchg to release lock, for future protection and to
keep gcc from acting clever.
2007-10-01 20:43:15 +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 4721271961 use larger, allocated cpu stacks 2007-09-27 19:32:43 +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 818fc0125e replace setjmp/longjmp with swtch 2007-08-28 12:48:33 +00:00
rsc 07090dd705 Remove struct uinode.
Remove type arg to mknod (assume T_DEV).
2007-08-24 20:54:23 +00:00
rsc 97ac612fb1 nits 2007-08-24 20:28:08 +00:00
rtm 2036534834 add missing iput() at end of _namei() 2007-08-24 14:56:17 +00:00
rsc b1fb19b6df Use parent pointer instead of ppid. 2007-08-23 14:40:30 +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 b6095304b7 Make cp a magic symbol. 2007-08-10 16:37:27 +00:00
rsc 7366e042d9 save process name for debugging 2007-08-08 08:38:11 +00:00
rsc 31085bb416 more comments 2006-09-07 14:12:30 +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
rtm 18432ed5ed nits 2006-08-29 21:35:30 +00:00
rtm 7a37578e9e clear killed flag in exit
idecref cwd in exit
2006-08-29 19:59:52 +00:00
rtm dfcc5b997c prune unneeded panics and debug output 2006-08-29 19:06:37 +00:00
rtm ceb0e42796 proc[0] can sleep(), at least after it gets to main00()
proc[0] calls iget(rootdev, 1) before forking init
2006-08-16 01:56:00 +00:00
rtm 350e63f7a9 no more proc[] entry per cpu for idle loop
each cpu[] has its own gdt and tss
no per-proc gdt or tss, re-write cpu's in scheduler (you win, cliff)
main0() switches to cpu[0].mpstack
2006-08-15 22:18:20 +00:00
kaashoek e958c538fa commented out code for cwd 2006-08-15 15:53:46 +00:00
rtm 5be0039ce9 interrupts could be recursive since lapic_eoi() called before rti
so fast interrupts overflow the kernel stack
fix: cli() before lapic_eoi()
2006-08-10 22:08:14 +00:00
rtm 0e84a0ec6e fix race in holding() check in acquire()
give cpu1 a TSS and gdt for when it enters scheduler()
and a pseudo proc[] entry for each cpu
cpu0 waits for each other cpu to start up
read() for files
2006-08-08 19:58:06 +00:00