Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757271Ab2BHTnG (ORCPT ); Wed, 8 Feb 2012 14:43:06 -0500 Received: from mx1.redhat.com ([209.132.183.28]:30325 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757190Ab2BHTnE (ORCPT ); Wed, 8 Feb 2012 14:43:04 -0500 Message-ID: <4F32D042.7080907@redhat.com> Date: Wed, 08 Feb 2012 19:42:58 +0000 From: Pedro Alves User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0) Gecko/20120131 Thunderbird/10.0 MIME-Version: 1.0 To: Oleg Nesterov 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 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> <20120208190250.GA23163@redhat.com> In-Reply-To: <20120208190250.GA23163@redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1670 Lines: 42 On 02/08/2012 07:02 PM, Oleg Nesterov wrote: > 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. Ah, right. It used to be the kernel copied the debug registers from parent->child, but they're always cleared in the child nowadays (since 72f674d203cd230426437cdcf7dd6f681dad8b0d). >> 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. Okay, I didn't know which it was that touched the memory, hence the "either". Thanks. Paired with the we-now-clear-debug-regs-on-clone thing, it makes sense. -- Pedro Alves -- 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/