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Postings from August 2011
On Tue Aug 30 09:46:32 2011, nicholas wrote:
> $ ./perl -Ilib dist/threads-shared/t/clone.t
> 1..34
> ok 1 - Loaded
> ok 2 - number
> ok 3 - string
> ok 4 - 1 arg
> ok 5 - cloned string
> ok 6 - cloned scalar ref
> ok 7 - cloned scalar ref in thread
> ok 8 - Circular ref typ
> ok 9 - Circular ref
> ok 10 - Circular ref in thread
> Attempt to free unreferenced scalar: SV 0x91eeb0, Perl interpreter:
> 0x973840 at dist/threads-shared/t/clone.t line 86.
> Attempt to free unreferenced scalar: SV 0x91eec8, Perl interpreter:
> 0x973840 at dist/threads-shared/t/clone.t line 86.
> Attempt to free unreferenced scalar: SV 0x8d15b8, Perl interpreter:
> 0x973840 at dist/threads-shared/t/clone.t line 86.
> ok 11 - Cloned circular refs from thread
> ok 12 - Cloned array
> ok 13 - Clone mod
> ok 14 - Original array
> ok 15 - Clone mod in thread
> Attempt to free unreferenced scalar: SV 0x902d18, Perl interpreter:
> 0x74ce60 at dist/threads-shared/t/clone.t line 108.
> Attempt to free unreferenced scalar: SV 0x902d30, Perl interpreter:
> 0x74ce60 at dist/threads-shared/t/clone.t line 108.
> Attempt to free unreferenced scalar: SV 0x977fd8, Perl interpreter:
> 0x74ce60 at dist/threads-shared/t/clone.t line 108.
> ok 16 - Clone mod from thread
>
> [etc, no more warnings]
>
> The test case resists most attempts to simplify it - removing
> seemingly
> unrelated earlier blocks from the test case removes the warning.
I’ve reduced it to this:
use threads;
use threads::shared;
use Devel::Peek;
Dump *DB::args;
{
my %hsh = ('' => undef);
eval {
shared_clone(%hsh);
};
}
my $bar :shared = shared_clone($x);
#{
my $foo :shared;
$foo = shared_clone(\$foo);
threads->create(sub {
Dump *DB::args;
undef $foo
})->join();
#}
__END__
Those Dump statements I’m using for debugging.
So far I’ve ascertained that:
1) The first unreferenced scalar is an entry in @DB::args that has
already been used as an AV by the time the thread is started.
2) It gets cloned by sv_dup (not _inc) when the thread starts, so it
ends up in param->unreferenced, and hence on the temps stack in the thread.
3) When the thread exits, cv_undef ends up freeing it, because it is
somehow reused as a padlist, even though it’s on the temps stack still
with a refcount of one. I don’t know which cv it is.
4) When the temps are freed, we get ‘Attempt to freed unreferenced scalar’.
Any ideas where to look now?
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