From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757844Ab1E3TY5 (ORCPT ); Mon, 30 May 2011 15:24:57 -0400 Received: from mx1.redhat.com ([209.132.183.28]:6292 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751317Ab1E3TY4 (ORCPT ); Mon, 30 May 2011 15:24:56 -0400 Date: Mon, 30 May 2011 21:22:55 +0200 From: Oleg Nesterov To: Tejun Heo Cc: Denys Vlasenko , jan.kratochvil@redhat.com, linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, akpm@linux-foundation.org, indan@nul.nu, bdonlan@gmail.com Subject: Re: [PATCH 03/10] ptrace: implement PTRACE_SEIZE Message-ID: <20110530192255.GB20616@redhat.com> References: <1305569849-10448-1-git-send-email-tj@kernel.org> <1305569849-10448-4-git-send-email-tj@kernel.org> <201105180240.56754.vda.linux@googlemail.com> <20110518095539.GU20624@htj.dyndns.org> <20110523124314.GA7232@redhat.com> <20110524102834.GC10334@htj.dyndns.org> <20110525182919.GB16575@redhat.com> <20110526091401.GD9715@htj.dyndns.org> <20110526150150.GD12525@redhat.com> <20110527182121.GA3212@mtj.dyndns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110527182121.GA3212@mtj.dyndns.org> 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 On 05/27, Tejun Heo wrote: > > On Thu, May 26, 2011 at 05:01:50PM +0200, Oleg Nesterov wrote: > > On 05/26, Tejun Heo wrote: > > > > Or we can change do_wait() to detach a zombie leader. In this case it > > > > is not clear what should we do if the debugger is the real parent. > > > > Perhaps do_wait() should do the same: detach a leader (but not reap). > > > > When the last thread does, the real parent will be notified again. > > > > IOW, wait(tgid) can succeed twice. > > > > > > Just letting PTRACE_DETACH work for zombies sounds much simpler to me. > > > > Probably, but please note we have to modify do_wait() anyway. Otherwise > > in general the tracer simply can not know the tracee has exited. IOW, > > waitpid(zombie_leader_pid, WEXITED) should succeed without reaping if > > delay_group_leader(), then the tracer can do PTRACE_DETACH. But this is > > not symmetrical with sub-thread zombies. > > Yes, complicated. The task/process duality conflicts. wait(2) > already is different for group leader and succeeds twice for the > ptracer and the real parent (when they're different). > > If we relocate ptrace group leader zombie wait, as suggested, such > that it waits for the task itself rather than the whole group, we > would be taking away a feature - ptracer waiting for the whole process Partly yes. The parent will be notified again and it can do another wait() later. The problem is, this is confusing. > I think group leader wait becoming asymmetrical with sub-thread > zombies is perfectly fine - it already is. Well, probably yes... But why? We have to change do_wait() anyway to report the death of the leader. If we do this, we can detach it as well. > But would it be okay to > change ptrace wait(2) on group leader to wait for the task itself > rather than the whole group? Yes, this is iffy. Simply because this can confuse the userspace. May be we can add W_PTRACED_THREAD_EXITED? (should be used instead of WEXITED by ptracer). Or, perhaps WEXITED should succeed but put something special into status? Oleg.