Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753224Ab2BHTJW (ORCPT ); Wed, 8 Feb 2012 14:09:22 -0500 Received: from mx1.redhat.com ([209.132.183.28]:56739 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753362Ab2BHTJV (ORCPT ); Wed, 8 Feb 2012 14:09:21 -0500 Date: Wed, 8 Feb 2012 20:02:50 +0100 From: Oleg Nesterov To: Pedro Alves Cc: Cyrill Gorcunov , 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: <20120208190250.GA23163@redhat.com> References: <20120203162519.GB805@google.com> <4F2C0F13.70709@parallels.com> <20120203165132.GA5636@redhat.com> <4F31848A.9070406@redhat.com> <20120207205639.GJ27437@moon> <4F31945B.5000304@redhat.com> <20120207215121.GB29773@moon> <4F326AF7.3060203@redhat.com> <20120208173103.GA20853@redhat.com> <4F32B924.8080407@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4F32B924.8080407@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1705 Lines: 46 On 02/08, Pedro Alves wrote: > > On 02/08/2012 05:31 PM, Oleg Nesterov wrote: > > On 02/08, Pedro Alves wrote: > >> > >> I just tried it. This is &pthread->tid in glibc/libpthread, so with debug > >> info it's easy to figure out where to set the watchpoint manually with gdb > >> without asking the kernel. Doesn't work. ptrace doesn't show any trap > >> for the kernel writes. > > > > The tracee simply can't report this trap. it is already dead ;) and > > hw breakpoint (used by ptrace) is "pinned" to the thread. > > Right, as I said. :-) I saw that a watchpoint trap isn't reported either > for the CLONE_CHILD_SETTID case (that is, within clone, when the kernel > writes the tid to the memory address passed in to the clone syscall). Yes. But in this case the new thread has no bps even if it is auto- attached. IOW, I think that hw bp can detect the write from the kernel space, but I didn't check. > I wouldn't have been surprised to see the trap in userspace in either > the parent It would be just wrong. Please note that it is child, not parent, who does the write. If only I understood why do we need CLONE_CHILD_SETTID... at least I certainly do not understand why glibc translates fork() into clone(CLONE_CHILD_SETTID) on my system. The child write into its memory, the parent can't see this change. IIRC, initially CLONE_CHILD_SETTID wrote child->pid into the parent's memory, and even before the child was actually created. Oleg. -- 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/