Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756934Ab2BGVv1 (ORCPT ); Tue, 7 Feb 2012 16:51:27 -0500 Received: from mail-bk0-f46.google.com ([209.85.214.46]:45105 "EHLO mail-bk0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756338Ab2BGVv0 (ORCPT ); Tue, 7 Feb 2012 16:51:26 -0500 Date: Wed, 8 Feb 2012 01:51:21 +0400 From: Cyrill Gorcunov To: Pedro Alves Cc: Oleg Nesterov , Pavel Emelyanov , Jan Kratochvil , Tejun Heo , Andrew Vagin , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH] ptrace: add ability to get clear_tid_address Message-ID: <20120207215121.GB29773@moon> References: <1328267483-3225342-1-git-send-email-avagin@openvz.org> <20120203162519.GB805@google.com> <4F2C0F13.70709@parallels.com> <20120203165132.GA5636@redhat.com> <4F31848A.9070406@redhat.com> <20120207205639.GJ27437@moon> <4F31945B.5000304@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4F31945B.5000304@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1913 Lines: 45 On Tue, Feb 07, 2012 at 09:15:07PM +0000, Pedro Alves wrote: > On 02/07/2012 08:56 PM, Cyrill Gorcunov wrote: > > On Tue, Feb 07, 2012 at 08:07:38PM +0000, Pedro Alves wrote: > >> On 02/03/2012 04:51 PM, Oleg Nesterov wrote: > >>> On 02/03, Pavel Emelyanov wrote: > >> > >>>> Because there's no need for current to get this value of himself, but can > >>>> be useful for e.g. gdb. > >>> > >>> OK, perhaps this makes sense, I do not know. > >>> > >>> Jan, Pedro, do you think gdb can use PTRACE_GET_TID_ADDRESS (returns > >>> tracee->clear_child_tid) ? > >> > >> Off hand, I'm not picturing a use. But that may well just mean I'm lacking > >> imagination. Andrew, Pavel, did you have a particular idea in mind when you > >> said it may be useful for debugging a multithreading program / gdb? > >> > > > > Might not we set up hw watchpoint on this address and get interrupt > > before pthread-join will find it? (To be fair I'm not sure if such > > trick will work, didn't test ;) > > For a debugger wanting to know when a pthread_join was about to return? > Might be simpler to put a breakpoint (or stap probe, or some such) inside > pthread_join. Yes, could be, but it means you have to install pthread debug libs, right? (have no idea actually since I personally use debug printing instead of breakpoints). > It's the kernel that writes to this address, so I've no > idea if the watchpoint trap ends up visible on userspace. Which thread > would it be reported to, given that this is cleared when the child > is gone, I have no idea either. Yeah, need some help from someone who wrote hw-breakpoints support in kernel (i don't remember the details). Cyrill -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/