Newsgroups: php.internals
Path: news.php.net
Xref: news.php.net php.internals:67745
Mailing-List: contact internals-help@lists.php.net; run by ezmlm
Received-SPF: pass (pb1.pair.com: domain gmail.com designates 209.85.220.182 as permitted sender)
MIME-Version: 1.0
In-Reply-To: <CABBJUpcz3g6sWZR_K+p6fe0_nP7YZqW1XgZ54ktgA1KgD6xchg@mail.gmail.com>
References: <CAAyV7nEx32ni3J0dOQprJCXd=4D06XwS0-Ro811OtqmPeEyYJg@mail.gmail.com>
	<CABBJUpcz3g6sWZR_K+p6fe0_nP7YZqW1XgZ54ktgA1KgD6xchg@mail.gmail.com>
Date: Thu, 20 Jun 2013 08:12:25 -0400
Message-ID: <CAAyV7nGFnDOzfR9B8PfgnbbG70ZVdwzx0XfZak9LoKuTSOkvEQ@mail.gmail.com>
To: Laruence <laruence@php.net>
Cc: "internals@lists.php.net" <internals@lists.php.net>
Content-Type: multipart/alternative; boundary=089e0111d92401fe8904df94dc1a
Subject: Re: [PHP-DEV] Disabling the GC during shutdown
From: ircmaxell@gmail.com (Anthony Ferrara)

--089e0111d92401fe8904df94dc1a
Content-Type: text/plain; charset=ISO-8859-1

Laruence,


    Sorry,  but I don't this this explain is right.
>
>     if there is more than one refcount to a zval, then it should never be
> freed
>
>     and if a zval is freed, then it must also be removed from the gc roots.
>

The point here is that the GC is run *while* the zval is being freed.

Check out the backtrace here: https://bugs.php.net/bug.php?id=64827 ,
specifically zval pointer 0x272afb8

It appears 4 times recursively being passed into
zend_objects_free_object_storage before the GC is triggered and it
segfaults.


>     according to your explain,  the gc segfault while walking through
> a hashtable of a object.
>

Yes, that is what's happening here. zval_mark_grey() is trying to walk
through the object's hash table, but the first bucket is already freed, so
when it tries to access it bad things happen.


>     but that doesn't make any sense, since if it segfault in walking,
> then it should also segfault when trying to free the hash table later
> while dtor the object.
>

That's the point. The dtor is already happening on that object when the GC
tries to run over it again.


>     disable GC in shutdown is okey for me. but that is just try to
> cover the bug somewhere in the refcount handler.. not the right fix.
>

Looking back through, we still have the problem where we can't null out the
zval before destructing the object (like we do with arrays) to prevent
this. That's why I suggested one alternative would be to modify
zend_objects_store_del_ref_by_handle_ex to also accept the zval, so it can
be nulled if the object is going to be freed.

However, we could set the object's bucket to invalid in the delref handler
before we call free_storage. And then modify the GC to check for a valid
bucket... I'll try that patch today to see if it solves the original issue
(as it shouldn't result in an API change either)...

Thanks for the thoughts...

Anthony

--089e0111d92401fe8904df94dc1a--