From: Simon Strandgaard <0bz63fz3m1qt3001@...>
Date: 2003-07-02T22:18:15+09:00
Subject: Re: rb_gc() and scan stack

On Wed, 02 Jul 2003 22:17:41 +0900, Mauricio Fern�ndez wrote:

> On Wed, Jul 02, 2003 at 07:52:17AM +0900, Simon Strandgaard wrote:
>> I were experimenting with Init_stack, when I discovered a flaw in my mind.
>> 
>> I thought that: local variables on stack would be marked in the
>> mark-phase and therefore survive the sweep-phase.
>> 
>> But my instance is being destroyed.. why ?
>> 
>> Can someone enligthen me ?
> [...] 
>> int main(int argc, char **argv) {
>>     ruby_init();
>>     Init_dummy();
>>     test();
>>     return 0;
>> }
> 
> Could you please check if adding one stack frame in between works?
> If it does, it'd mean it's exactly the same issue described in the links
> I provided some time ago, that is, that ruby_init is capturing the
> current SP to record the beginning of the stack, but you're storing
> VALUEs in a stack frame above.
> 
> Please test adding the following to your code:
[snip] 
> 
> Cannot check now as my primary machine has no connection and the one
> I'm writing from has no ruby...


It doesn't help.. my main now looks like this:

> expand -t4 main.c | tail -n12
void foo() {
    int a, b, c, d, e, f; /* just to take place in the stack */
    a = b = c = d = e = f = 1; /* don't get warnings when -Wall */
    test();
}

int main(int argc, char **argv) {
    ruby_init();
    Init_dummy();
    foo();
    return 0;
}
>

Output is the same:
> ./a.out
alloc
initialize
1
#<Dummy:0x8103b38>
free
2
ruby: [BUG] Segmentation fault
ruby 1.8.0 (2003-06-23) [i386-freebsd5.0]

Abort (core dumped)
>

I tried drawing on some paper, what is happening..
My initial drawing were wrong.. Now I understand.
No problemo:

The stack grows againts right.

'.' = empty stack range (unkown to GC).
'R' = stack range scanned by GC.
left-most-R == rb_gc_stack_end.
right-most-R == rb_gc_stack_begin  (set by Init_stack).
'|' = current stack pointer.
'i' = where the instance is on the stack.


--[ 0 ]-----------------
top/hi [.....................] bottom/lo
        |

--[ 1 ]-----------------
entering ruby_init()
top/hi [.....................] bottom/lo
          |

--[ 2 ]-----------------
leaving ruby_init()
top/hi [RRRRR................] bottom/lo
        |

--[ 3 ]-----------------
entering Init_dummy()
top/hi [RRRRR................] bottom/lo
          |

--[ 4 ]-----------------
leaving Init_dummy()
top/hi [RRRRR................] bottom/lo
        |

--[ 5 ]-----------------
entering test()
top/hi [RRRRR................] bottom/lo
               |

--[ 6 ]-----------------
instance = Dummy.new
The instance is not within the mark-area, therefore it dies!
top/hi [RRRRR..i.............] bottom/lo
               |

--[ 7 ]-----------------
rb_gc(); /* first time */

The instance is not within the RRRRR range... and
Ruby is therefore unable to mark the instance.
Thus it dies!

top/hi [RRRRR................] bottom/lo
        |


If we redo from step6 with Init_stack(begin).. with a
lower begin address, then things will work.

--[ 6a ]-----------------
Init_stack(current_stack_ptr)
top/hi [RRRRRRRR.............] bottom/lo
               |

--[ 7a ]-----------------
instance = Dummy.new
This time the instance is within the area, and will thus survive
top/hi [RRRRRRRR.............] bottom/lo
               |

--[ 8a ]-----------------
rb_gc(); /* first time: this time we survie */
top/hi [RRRRRRRR.............] bottom/lo
               |

--[ 9a ]-----------------
i = nil
top/hi [RRRRRRRR.............] bottom/lo
               |

--[ 10a ]-----------------
rb_gc(); /* second time: this time we die */
top/hi [RRRRRRRR.............] bottom/lo
               |

--[ 11a ]-----------------
leaving test();
top/hi [RRRRRRRR.............] bottom/lo
        |


--
Simon Strandgaard