rtm wrote:
> Why does acquire() call cpuid()? Why does release() call cpuid()?
The cpuid in acquire is redundant with the cmpxchg, as you said.
I have removed the cpuid from acquire.
The cpuid in release is actually doing something important,
but not on the hardware. It keeps gcc from reordering the
lock->locked assignment above the other two during optimization.
(Not that current gcc -O2 would choose to do that, but it is allowed to.)
I have replaced the cpuid in release with a "gcc barrier" that
keeps gcc from moving things around but has no hardware effect.
On a related note, I don't think the cpuid in mpmain is necessary,
for the same reason that the cpuid wasn't needed in release.
As to the question of whether
acquire();
x = protected;
release();
might read protected after release(), I still haven't convinced
myself whether it can. I'll put the cpuid back into release if
we determine that it can.
Russ
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.
some memory references aren't actually valid
and gcc is a bit too smart in parsing (%0) and
a bit too dumb in knowing which memory refs
are not valid. Using "r" seems to constrain gcc
to use a register as in (%eax) and not substitute
a memory reference like (-36(%ebp)).
No one really understands these things.
fix acquire() to cli() *before* incrementing nlock
make T_SYSCALL a trap gate, not an interrupt gate
sadly, various crashes if you hold down a keyboard key...
nesting cli/sti: release shouldn't always enable interrupts
separate setup of lapic from starting of other cpus, so cpu() works earlier
flag to disable locking in console output
make locks work even when curproc==0
(still crashes in clock interrupt)
Linux 2.4 box using gcc 3.4.6 don't seem to follow the same
conventions as the i386-jos-elf-gcc compilers.
Can run make 'TOOLPREFIX=' or edit the Makefile.
curproc[cpu()] can now be NULL, indicating that no proc is running.
This seemed safer to me than having curproc[0] and curproc[1]
both pointing at proc[0] potentially.
The old implementation of swtch depended on the stack frame layout
used inside swtch being okay to return from on the other stack
(exactly the V6 you are not expected to understand this).
It also could be called in two contexts: at boot time, to schedule
the very first process, and later, on behalf of a process, to sleep
or schedule some other process.
I split this into two functions: scheduler and swtch.
The scheduler is now a separate never-returning function, invoked
by each cpu once set up. The scheduler looks like:
scheduler() {
setjmp(cpu.context);
pick proc to schedule
blah blah blah
longjmp(proc.context)
}
The new swtch is intended to be called only when curproc[cpu()] is not NULL,
that is, only on behalf of a user proc. It does:
swtch() {
if(setjmp(proc.context) == 0)
longjmp(cpu.context)
}
to save the current proc context and then jump over to the scheduler,
running on the cpu stack.
Similarly the system call stubs are now in assembly in usys.S to avoid
needing to know the details of stack frame layout used by the compiler.
Also various changes in the debugging prints.
