Commit graph

472 commits

Author SHA1 Message Date
rsc b5dcebdbeb better lapic writes, suggested by cliff 2007-09-27 19:33:46 +00:00
rsc 4721271961 use larger, allocated cpu stacks 2007-09-27 19:32:43 +00:00
rsc 0fe118f3f6 don't call it ss - that's the stack segment 2007-09-27 16:47:50 +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 75506c6655 use console lock 2007-09-27 12:29:25 +00:00
rsc d522571068 make slow bigdir last test 2007-09-27 12:29:06 +00:00
rsc ad12b487b5 changes since two days ago:
drop , address=0xf0000 from romimage line.
newer bochs has a 128k bios that it loads elsewhere.
so let bochs decide where the romimage goes.

change cpu quantum to 1 (default is 5, max is 16)
in an attempt to provoke more races.  only provokes
them slightly more frequently, may not be worth
the slowdown.
2007-09-27 11:27:04 +00:00
rsc b30ab3f5af use standard bios location 2007-09-27 05:14:25 +00:00
rsc 666f58c711 believe it or not, this was working
the macro expansion of "char *cp;" turned into
char *(curproc[cpu()]);  which declares a dynamically
sized array of char* called curproc.

so then &cp == &(curproc[cpu()]) was actually a
stack variable as "expected".  it was one past the
end of the array, but the implicit alloca allocated
more than was necessary.

do not tell me that making cp a #define was a bad idea.
there are worse problems to fix.  more on that later.
2007-09-27 05:13:10 +00:00
rsc 90d975e9c8 comment bochs nonsense 2007-09-26 23:32:47 +00:00
rsc fbaa7b428e various comment and print tweaks 2007-09-26 23:32:00 +00:00
rsc 56c1a151d2 debugging prints 2007-09-26 23:24:23 +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
rtm 355073ea9e oops, interrupts on in syscall traps doesn't work after all 2007-09-25 16:15:05 +00:00
rtm 3eda2714e6 tell SETGATE to leave interrupts on for T_SYSCALL
panic if unknown fault with CPL=0 (i.e. in kernel)
2007-09-25 15:23:44 +00:00
nelhage d7a90d3b5c This should fix building on FreeBSD 2007-09-19 23:49:52 +00:00
nelhage 6197c04208 Fix compilation on 64-bit machines (thanks to andersk for patch) 2007-09-18 00:41:34 +00:00
rtm 411ee74127 fix comments 2007-09-15 20:05:47 +00:00
rsc e9f8419fbe sh 2007-09-05 15:55:43 +00:00
rtm ab4cedb593 continuous quality management 2007-08-31 19:55:27 +00:00
rsc ef2185247d symlink implementation 2007-08-30 18:36:38 +00:00
rsc e51ae8a272 do not toss .ps 2007-08-30 18:33:48 +00:00
rsc 9863fea78c clumsy cd 2007-08-30 18:30:26 +00:00
rtm fbd8857d4d make new Homework 8 work 2007-08-30 18:21:35 +00:00
rsc 37321196a4 oops - broke circular buffer 2007-08-30 18:20:53 +00:00
rsc 71d5bf4d08 oops - broke arg counting 2007-08-30 18:19:52 +00:00
rtm de1329dda2 longjmp -> swtch in comments 2007-08-30 17:39:56 +00:00
rsc 05109382d6 tweak 2007-08-30 14:12:19 +00:00
rsc 42f3c3f7be DO NOT MAIL: xv6-rev1 2007-08-30 14:11:21 +00:00
rsc 55401f2adb final xv6 for 2007 2007-08-30 14:09:14 +00:00
rsc febdc07c35 bootothers now in main 2007-08-29 19:20:49 +00:00
rtm bc54fa3915 spelling 2007-08-29 18:18:57 +00:00
rsc bf2932a686 final nits 2007-08-28 19:39:49 +00:00
rsc 68a2d5373d match README 2007-08-28 19:30:29 +00:00
rsc f040060092 nits 2007-08-28 19:30:23 +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 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 c1b100e930 nits 2007-08-28 18:23:48 +00:00
rsc 3e1eaf226d fix offsets 2007-08-28 18:04:43 +00:00
rsc a491dba00c more cmain -> bootmain 2007-08-28 18:02:49 +00:00
rsc d844f0f9d9 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.
2007-08-28 17:49:49 +00:00
rsc e3f271e880 oops 2007-08-28 17:48:44 +00:00
rsc c35c064e04 cmain -> bootmain 2007-08-28 13:01:10 +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 b52dea08bc never returns! 2007-08-28 05:19:45 +00:00
rsc 0c931af50f nits 2007-08-28 05:01:04 +00:00