27 Mar
2016
27 Mar
'16
11:07 p.m.
On 2016-03-27 23:49, jnc(a)mercury.lcs.mit.edu (Noel Chiappa) wrote:
Hmm. A good point. If you depend on WAIT waking you up at an interrupt,
then you need SPL to block here. But this also means that you must be at
SPL 7 before any of this, otherwise you are still exposed to this
problem (nothing says that the interrupt won't happen before the SPL as
well).
In general, I would say that this is not the way I would write code, but
checking in RSX and 2.11BSD I can tell that RSX do not use this pattern,
and does a WAIT without any SPL, while 2.11BSD do an SPL 0 followed by
WAIT. And the routine in 2.11BSD is also called at SPL 7.
So obviously, both ways have been done, and 2.11BSD will work
potentially with a slight degration if the SPL do not block interrupts.
It will still work fine, as you will, at a minimum, get an interrupt at
the next clock tick, which will wake it up. But it might possibly be
sitting in a WAIT slightly longer than required.
RSX in fact just loops after the WAIT. If an interrupt should cause the
system to be able to do something more productive, it will not return to
the idle loop. So yes, it also detects in the interrupt exit processing,
that it was/is in the idle loop.
I still haven't had time to investigate properly. But at least processor
and chip manuals do not say that SPL will block interrupts. But that is
no guarantee that it don't in reality.
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt(a)softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
From: Johnny Billquist
It would also be interesting if anyone can come
up with a good reason
why SPL should work that way.
So that when doing:
SPL 0
WAIT
you don't lose by having the interrupt happen between the SPL and the WAIT? 
28 Mar
28 Mar
1:37 p.m.
New subject: PDP-11/70 SPL
On Mon, 28 Mar 2016, Johnny Billquist wrote:
SPL 7 was only used by the clock interrupt, and levels 1-3 were never used
in Unix. spl4() was slow devices, spl5() was fast devices, but I can't
remember how spl6() was used (blame my fading memory).
In other words, spl7() was diable all interrupts, spl0() was enable all
interrupts, and spl4()-spl6() meant "you'd better be doing something more
important than what I'm doing" (per The Book).
And that was the beauty; even the clock couldn't interrupt, so bye-bye all
scheduling... And the HALT key, being implemented as an interrupt,
as well...
I'm just starting to think that this "bug" was left undocumented on
porpoise, so perhaps someone read the engineering drawings?
SPL 0
WAIT
you don't lose by having the interrupt happen between the SPL and the
WAIT?
Hmm. A good point. If you depend on WAIT waking you up at an interrupt,
then you need SPL to block here. But this also means that you must be at
SPL 7 before any of this, otherwise you are still exposed to this
problem (nothing says that the interrupt won't happen before the SPL as
well). In general, I would say that this is not the way I
would write code, but
checking in RSX and 2.11BSD I can tell that RSX do not use this pattern,
and does a WAIT without any SPL, while 2.11BSD do an SPL 0 followed by
WAIT. And the routine in 2.11BSD is also called at SPL 7.
See above comment; perhaps they believed the handbook? Heck, even the
published Unibus spec was known to be wrong, in order to keep 3rd-party
kit out of it (it was something subtle to do with buss timing, so
sometimes the card worked, and sometimes it didn't, doing wonders for
your reputation).
So obviously, both ways have been done, and 2.11BSD
will work
potentially with a slight degration if the SPL do not block interrupts.
It will still work fine, as you will, at a minimum, get an interrupt at
the next clock tick, which will wake it up. But it might possibly be
sitting in a WAIT slightly longer than required.
Slightly longer? I think it was Lions himself who used to teach us that a
lost interrupt is nasty :-(
RSX in fact just loops after the WAIT. If an interrupt
should cause the
system to be able to do something more productive, it will not return to
the idle loop. So yes, it also detects in the interrupt exit processing,
that it was/is in the idle loop.
Also keep in mind that the company who wrote RSX-11 also designed the
hardware, and thus had a vested interest. Anyone here remember overlapped
seeks on the RK-11 failing under Unix, but not under RSX, so therefore it
must be Unix's problem? I hammered those buggers until they supplied some
new diag routines that pretty much demonstrated the bug... RSX of course
never used overlapped seeks on the RK-11, so MAINDEC never tested for it.
This has nothing to do with how "SPL 0; WAIT" etc was implemented in Unix;
we are talking hardware here.
I still haven't had time to investigate properly.
But at least processor
and chip manuals do not say that SPL will block interrupts. But that is
no guarantee that it don't in reality.
Sigh; again, there are far too many witnesses that showed that it did; not
wishing to be offensive or anything, but did you work for DEC back then?
Ah; John Dodson: are you here? Did you try that little program on your
11/70?
Ian Johnson, are you here, when I "accidentally" crashed your machine, and
after I fessed up to it, you referred to me as a vulgar term for the
female genitalia, then promptly hung up?
I think that Kev Hill can attest to this, and I know that Kevin Dawson (I
think) tried it on my /40 as well; then again, his "outside" group were
always trying to hack into us, and my job was to keep the barbarians away
from the gate (which is how I got my first job at UNSW: it was better to
have me inside than outside...).
What about you, MarioE, ChrisT, and RonDeJ? (And UNSW actually employed
most of 'em.) It's quite likely that ChrisM, GregR, and PeterI knew about
it as well, along with possibly anyone on the old AUUG list, where the
exploit was published, IIRC.
Christ, but this is starting to sound like some religion or other.
Yes, Warren, I need to know that should I write that history, that we
won't both get sued; how long is the Statute of Limitations anyway?
--
Dave Horsfall DTM (VK2KFU) "Those who don't understand security will
suffer."
3:18 p.m.
New subject: PDP-11/70 SPL
Dave Horsfall scripsit:
Ian Johnson, are you here, when I
"accidentally" crashed your machine, and
after I fessed up to it, you referred to me as a vulgar term for the
female genitalia, then promptly hung up?
If he was a Brit or Ozite, he was merely calling you a jerk or idiot;
the word has a different connotation there, and is most often applied
to men rather than women. (This is probably calqued from French _con_,
which has shifted meaning completely.)
Christ, but this is starting to sound like some
religion or other.
<http://untroubled.org/articles/gospel-of-tux.txt>
Yes, Warren, I need to know that should I write that
history, that we
won't both get sued; how long is the Statute of Limitations anyway?
Tell the truth and shame the Devil, and fear no libel. Who's left to
sue you anyway?
--
John Cowan http://www.ccil.org/~cowan cowan(a)ccil.org
Income tax, if I may be pardoned for saying so, is a tax on income.
--Lord Macnaghten (1901)
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