[M3devel] cvsup fix refinements

Fri Mar 19 19:47:46 CET 2010

How about DoesForkExist, DoesForkInheritAllThreads?

From: jay.krell at cornell.edu
To: hosking at cs.purdue.edu
Date: Fri, 19 Mar 2010 18:15:34 +0000
CC: m3devel at elegosoft.com
Subject: Re: [M3devel] cvsup fix refinements

RTProcess is fine.
Unix is a little wierd because I don't think it should do anything if using userthreads.
Unix implies a fairly thin wrapper?

 - Jay

Subject: Re: [M3devel] cvsup fix refinements
From: hosking at cs.purdue.edu
Date: Fri, 19 Mar 2010 14:13:32 -0400
CC: m3devel at elegosoft.com
To: jay.krell at cornell.edu

Why not put it into RTProcess?Or into Unix?
I want to avoid confusion between Thread.Fork (fork a thread) and process fork.
On 19 Mar 2010, at 14:08, Jay K wrote: > could go in any of ThreadF, RTOS, RTProcess?

Or RTThread. Or almost anywhere.

Some typos in the diffs:
 in a comment, ThreadF__ should be RTOS__
 function PThreadForkHandler named inconsistently,
  should be PThreadAtForkHandler

Again, "PThread" appears in these names to indicate
their meaning is not portable. Their existance is, but not their meaning.
We have no way of saying "only call this function for pthreads",
instead as I understand, we provide a portable interface with
drastically varying semantics, such as "do something" vs. "do nothing".

Given that Init in ThreadPThread.m3 is private, we could probably
take the stackbase parameter from the caller instead.

The call to AtFork should probably follow InitWithStackbase,
in case it fails under low memory, the Die/assert more likely to "work".

I'm not completely happy making RTOS public.
 So maybe ThreadF?
 I had gone to RTOS because SetNextForkWillExec required LockHeap/UnlockHeap.
 But for now they don't exist.

Maybe a new interface? ThreadPThread.i3, but is still
present on all platforms, so mostly portable can call it.

ThreadPThread.AtFork?
PThread.AtFork? That seems right.

 - Jay

From: jay.krell at cornell.edu
To: m3devel at elegosoft.com
Date: Fri, 19 Mar 2010 17:21:13 +0000
Subject: Re: [M3devel] cvsup fix refinements

diffs attached

For now I've removed the Get/SetNextForkWillExec.
 I also think truly get/set is right, not inc/dec.
 It was confusing enough to interpret and initialize
 the value. And it is also not clear what the default
 should be. The usual behavor is exec does follow fork.
 The safer default if the handlers work ok is to assume
 exec will not follow. It is a perf hint or a don't change
 how things were hint? Hint to speed up fork+exec or hint
 to avoid running the new code that might not work?

(For now I've removed the Get/SetNextForkWillExec.)
  They'd have to be implemented three times, and
  I had trouble defining them.
  Obviously Win32/userthreads just return a
  hardcoded true, but it is hard to explain
  from the caller of Get's point of view.

Maybe:
  SetNextForkWillExec
  GetNextForkWillExec

and then:
  PThreadAtForkNeeded() = GetNextForkWillExec = FALSE ?

That is,
 someone is who is calling fork and possibly exec,
 can immediately tell what these functions mean.

But the implementer of an atfork handler has to
  do quite a semantic translation I think.
  I wrote it backwards the first time. That either
  implies it is confusing, or I wasn't thinking.

If it really is a problem to run this stuff when exec
will follow, we should know quickly, as building cm3
is an aggressive user of this code.

This version has worked repeatedly on Darwin.
I didn't test user threads but it is structured such
as to make that obviously work.
  The cvsup changes are in pthreaad_atfork handlers,
  so no change for userthreads.

I didn't test on others but confidence is quite high now.

description of changes at least where they are hard to read:

m3core:
  Init split into Init and InitWithStackBase,
    We have to provide the old stack base in the child fork handler
    instead of estimating from the address of a local. I don't
    know what stack is used, maybe the caller of fork?
    i.e. we might have eaten a fair amount of stack and
    there could be arbitrary references above the current stack.

WeakRefFromRef split into WeakRefFromRef and StartWeakCleaner

There is a resulting double lock in WeakRefFromRef, every time.
Could instead copy/paste more code around, i.e.:
      EVAL Thread.Fork(NEW(Thread.Closure, apply := WeakCleaner));

 - Jay

From: jay.krell at cornell.edu
To: m3devel at elegosoft.com
Date: Fri, 19 Mar 2010 16:01:03 +0000
Subject: [M3devel] cvsup fix refinements

refinements:

new public functions in m3core:

TYPE PThreadForkHandler = PROCEDURE();

<* EXTERNAL RTOS__PThreadAtFork *>
PROCEDURE PThreadAtFork(prep, parent, child: PThreadForkHandler): INTEGER;

(That's not a change yet.)

PROCEDURE SetNextForkWillExec(value: BOOLEAN);

PROCEDURE GetNextForkWillExec(): BOOLEAN;

 These are only an optimization and should
 perhaps be removed? They should be used
 under LockHeap/UnlockHeap? which wasn't previously
 public. This way, existing fork/exec paths
 not affected, though maybe though might as well
 ought to be?

could go in any of ThreadF, RTOS, RTProcess?

Should be called under RTOS.LockHeap?
  Therefore I put in RTOS.
  Also helps that RTOS is exported from *Thread.m3.
RTOS not previously public.

Should Get/Set be Inc/Dec? (therefore just Inc with +1,-1?)
 I implemented them that way: true incs, false decs.

Or don't bother with them at all?

Furthermore, in RTCollector, instead of claiming the threads
aren't started, if they were started, restart them.
This makes more sense for the weak cleaner to me, and
seems reasonable for the other two.

Diffs not yet available.

 - Jay

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