Re: [Chicken-users] remove enable/disable interrupt flag

[Alan Post]

On Fri, Sep 30, 2011 at 09:30:13AM +0100, Alaric Snell-Pym wrote:
> Your other points about multiple signals sugget it should be a proper
> queue, not just a bitmask. Although I have a vague feeling that Unix was
> allowed to coalesce pending signals as it just used a bitmask itself...
> Meh, I dunno, I only look into signal handling occasionally!
> 

I was misremembering a statement from APUE (Advanced Programming in
the Unix Environment), which talks about signal queuing being
*permitted* by Posix, but not required.  And no one does it.
A test program on *BSD confirms that this operating system does not,
and Linux likely follows.  So we don't need that queue--I think I
managed to see Chicken drop one signal testing last night, but I've
not confirmed this yet and it wouldn't be a crisis should it
actually do so.  I haven't tried a more complex case I think Chicken
does suffer from, which is near-arrival of two separate signals.

At any rate, my suggestion of a signal queue was based on an
incorrect understanding of the signal mechanism and I withdraw it.


> > And that we also *must* handle
> > deferred signals before making another syscall, whether that syscall
> > happened from user code or whether we're making a syscall in service
> > to the need of the runtime.
> 
> The one tricky case I can think of is when the chicken signal handler
> needs more RAM (so the GC is invoked) and the GC needs to malloc things
> in the heap and the heap needs to grow, so a mmap (which is what sbrk
> seems to be called these days) occurs... Do you need it run before ALL
> syscalls, or just ones that might block on something the signal handler
> might be needed to relieve? AIUI mmap of /dev/zero (to allocate more
> empty heap) can't block on very much...
> 

I'm currently hoping that it can be run just before the ones that
block, which is an astute observation, thank you.  I'll probably
need to sit down with the signals one-by-one to be sure, but I
think this "weaker" constraint is fully correct.


> > I think is a fantastic outline of what needs to happen.  I will work on
> > a patch, with no guarantee of how fast I will be.
> 
> Tell me if I can help - I want to gain a deeper understanding of the
> Chicken scheduler!
> 

Thank you!

-Alan
-- 
.i ma'a lo bradi cu penmi gi'e du