From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <3bb7383c0179d2512acec9be881a79c5@gmx.de>
To: 9fans@9fans.net
Date: Sat, 30 Aug 2008 19:40:38 +0200
From: cinap_lenrek@gmx.de
In-Reply-To: <003239afe18d1b7672c1f71363ec9e64@coraid.com>
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Subject: Re: [9fans] notes and traps
Topicbox-Message-UUID: 0b923a6e-ead4-11e9-9d60-3106f5b1d025

> > I'm not claiming anyone to be wrong. I want to solve
> > this problem.
>
> i'm not sure there is a problem with notes.  but i really can't
> answer that question.  someone with a better understanding
> of what notes are supposed to be could answer this.
>
> > I also noted that i think this is the correct behavior in
> > the previous mail. My patch is on the other side inpostnote(),
> > where the note is put in the up->note[] array, not where
> > it decides to kill the proc.
>
> i'm not convinced that you've explained why this change is
> correct or why it could solve the problem. after all, the
> NNOTED array is only 5 entries long.  what if one gets 5
> user notes before the system note?

assuming now that up->note[] is filled up and up->nnote = 5.
the trap happens... we are in sys/src/9/pc/trap.c trpa():
we detect the fault and do a postnote(up, 1, "sys: trap: bla", NDebug)
postnote() will see that p->notified is 0 becasue we are not in the
note handler yet, so it will skip the:

	if(flag != NUser && (p->notify == 0 || p->notified))
		p->nnote = 0;

then it checks if here is any room to store the note, which is not
the case. then we set p->notepending and exit.

So now, our trap note got lost! We will *not* get killed then... and happily
process the pending 5 user notes.

With my suggested patch, it would have discarded all the notes
and *have* the NDebug note posted. Notify will pick it up and kill
us if here is no handler installed or if it happend while inside the
note handler.

> do you kill the process (isn't this how it works now?),

no, it would lose the note with the current implementation.

> make notes unreliable or block the sender?

yes, trowing usernotes away when something important as an
internal note happend to make sure it gets put in the note array
so we have a chance to process it:

	if(n->flag!=NUser && (up->notified || up->notify==0)){
		if(n->flag == NDebug)
			pprint("suicide: %s\n", n->msg);
		qunlock(&up->debug);
		pexit(n->msg, n->flag!=NDebug);
	}

This was already in the code (see postnote()).  The only thing that i
claim to change is that it doesnt matter if we are notified or not
when posting the note.  We check for it in notify() ayway so the
process *will* be terminated if it causes another trap while inside
its note handler.

> none of these
> options seems like an improvement to me.

losing the trap note and *not* handling it at all looks like
a bug to me.

> > >  sleeping in a note handler seems
> > > like it's pushing the limits to me.
> >
> > It doesnt matter if we sleep() here. You could do a
> > write() or any other syscall instead. Or do the looping
> > and wait for the timer interrupt.
>
> i know.  but sleep gives other processes a large window
> to deliver more notes.  what is the external note, by the
> way?  a keyboard interrupt?

External notes are the ones posted to /proc/pid/note[pg].
Like if you hit [DEL], rio writes "interrupt" the the programs
note file running in the window.

> > Being curious, why is sleep() in a note handler a bad
> > idea?
>
> as i see it (please chime in if you disagree), a process in
> a note handler is in a funny state.  the kernel treats this
> state as funny as it keeps up->notified around
> to note (ha!) this fact.

As i see it, up->notified just means we have put the process
into its note handler, and any further notes that are in the
up->note[] array (that are NUser) get not handled until the
user process calls noted().

Thats exactly how it is documented in notify(2):

Until the program calls noted, any further externally-generated notes
(e.g., hangup or alarm) *[1] will be held off, and any further notes
generated by erroneous behavior by the program (such as divide by
zero) *[2] will kill the program.

[1] that are, NUser notes
[2] NDebug for traps

> as a general rule, i would think that anything prolonging
> this funny state would tend to cause problems.

> > Dont tell me... But i cant think of any other way to catch
> > linux syscalls for now.
>
> without changing your mechanism, you could move the
> work out of the note handler and let the note handler
> just set state.

Not in this case. I need both notes to interrupt the linux
machine code (which i cant change) and to catch the
syscalls.

With the current handling, the only place where it would be save to
send (external) notes to a process from another is where i'm not in
linux user code (which can trigger a trap when its doing a syscall).
I dont generate traps in the linuxemu code.  (Which would kill me as
documneted in the manpage)

> for example, byron's rc ^c handler just set a global
> interrupted variable.  there were only three or four
> places where this variable was checked.  these locations
> were carefully vetted for longjmp safety. and lonjmp'ed
> back to the command loop (if interactive).  it doesn't
> sound like it would work well, but it worked perfectly.

The problem with notes and signals are that it can interrupt
your code in places where your data structures will be in an
inconsistent state so you would not be able to do anything
usefull here.

I agree that channels are the better and more deterministic
way to pass another process some information or notification,
because the "interruption" happens on well defined places
only.

But here are places where you need notes. For example if
you have to interrupt blocking systemcalls like in ioproc(),
or in my case, to get me out of the linux code to check if
here are linux signals pending and catching systemcalls.

> about the same time byron was working on rc, i wrote
> my own shell.  i wasn't convinced that byron's technique
> would work or would be more reliable than doing things
> from the signal handler.  ... experience says that i was
> wrong.
>
> there's also a special dodge of a note continuation handler
> for ape available.  this hints that the note interface might
> be pushed too far by ape.

Linuxemu doesnt use/need that special NSAVE NRSTR stuff.
Really, if here are linux signals all i do is put the information
to continue after the signal on the stack and transfer execution
back to linux code in the linux signal handler.

> - erik

--
cinap