From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!.POSTED.blaine.gmane.org!not-for-mail From: Alan Mackenzie Newsgroups: gmane.emacs.devel Subject: Re: Help please! To track down GC trying to free an already freed object. Date: Tue, 2 Apr 2019 20:42:37 +0000 Message-ID: <20190402204237.GB25792@ACM> References: <20190402112537.GA6212@ACM> <83r2akfnyh.fsf@gnu.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Injection-Info: blaine.gmane.org; posting-host="blaine.gmane.org:195.159.176.226"; logging-data="252859"; mail-complaints-to="usenet@blaine.gmane.org" User-Agent: Mutt/1.10.1 (2018-07-13) Cc: emacs-devel@gnu.org To: Eli Zaretskii Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Tue Apr 02 22:43:02 2019 Return-path: Envelope-to: ged-emacs-devel@m.gmane.org Original-Received: from lists.gnu.org ([209.51.188.17]) by blaine.gmane.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:256) (Exim 4.89) (envelope-from ) id 1hBQFK-0013fk-8V for ged-emacs-devel@m.gmane.org; Tue, 02 Apr 2019 22:43:02 +0200 Original-Received: from localhost ([127.0.0.1]:40167 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hBQFJ-0008Ov-67 for ged-emacs-devel@m.gmane.org; Tue, 02 Apr 2019 16:43:01 -0400 Original-Received: from eggs.gnu.org ([209.51.188.92]:37721) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hBQF8-0008Nn-HG for emacs-devel@gnu.org; Tue, 02 Apr 2019 16:42:51 -0400 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hBQF7-0006Qt-Cc for emacs-devel@gnu.org; Tue, 02 Apr 2019 16:42:50 -0400 Original-Received: from colin.muc.de ([193.149.48.1]:52438 helo=mail.muc.de) by eggs.gnu.org with smtp (Exim 4.71) (envelope-from ) id 1hBQF7-0006La-4k for emacs-devel@gnu.org; Tue, 02 Apr 2019 16:42:49 -0400 Original-Received: (qmail 8249 invoked by uid 3782); 2 Apr 2019 20:42:38 -0000 Original-Received: from acm.muc.de (p4FE15EFB.dip0.t-ipconnect.de [79.225.94.251]) by colin.muc.de (tmda-ofmipd) with ESMTP; Tue, 02 Apr 2019 22:42:37 +0200 Original-Received: (qmail 25939 invoked by uid 1000); 2 Apr 2019 20:42:37 -0000 Content-Disposition: inline In-Reply-To: <83r2akfnyh.fsf@gnu.org> X-Delivery-Agent: TMDA/1.1.12 (Macallan) X-Primary-Address: acm@muc.de X-detected-operating-system: by eggs.gnu.org: FreeBSD 9.x [fuzzy] X-Received-From: 193.149.48.1 X-BeenThere: emacs-devel@gnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: "Emacs development discussions." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Original-Sender: "Emacs-devel" Xref: news.gmane.org gmane.emacs.devel:234901 Archived-At: Hello, Eli. On Tue, Apr 02, 2019 at 18:04:22 +0300, Eli Zaretskii wrote: > > Date: Tue, 2 Apr 2019 11:25:37 +0000 > > From: Alan Mackenzie > > With GDB, I can break at the creation of this symbol-with-position > > object and again at its (first) freeing with this breakpoint: > > break setup_on_free_list if (v == 0x5555561d0450) > > . However, this isn't helping me to track down the Lisp object which > > still references this symbol-with-position. I've tried to find the > > address of Emacs's data segment, so as to be able to search through it > > for 0x5555561d0455 in GDB, but this doesn't feel like a very useful > > thing to do. > > Could somebody who has experience in this sort of thing please suggest > > how I might proceed with the debugging, or possibly offer me some other > > sort of help or hints. > The usual method of debugging such problems is described in etc/DEBUG, Apologies, I didn't see this. I read quite a bit of etc/DEBUG, but for some reason completely missed the bit about GC problems. > it basically uses the last_marked[] array. You start with the object > at last_marked[last_marked_index - 1], and go backwards (in circular > manner), comparing the objects you find in the array with those you > see in the call-stack frames that call mark_* functions. Just be very > careful when you print the objects; e.g., never use 'pp', because the > function it calls cannot handle marked objects. I'm having some difficult seeing the entire last_marked array with GDB. I will try to find a solution in the GDB manual. > If you already tried this, please ask more specific questions. No, I hadn't. I didn't know about last_marked. I'll see if I can get further with its help. Thanks! -- Alan Mackenzie (Nuremberg, Germany).