From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933662Ab3GWQnM (ORCPT ); Tue, 23 Jul 2013 12:43:12 -0400 Received: from mx1.redhat.com ([209.132.183.28]:14451 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932705Ab3GWQnL (ORCPT ); Tue, 23 Jul 2013 12:43:11 -0400 Date: Tue, 23 Jul 2013 18:38:06 +0200 From: Oleg Nesterov To: Mike Galbraith Cc: LKML Subject: Re: ptrace(PTRACE_ATTACH) [no intervering wait] ptrace(PTRACE_DETACH) may leave tracee stuck Message-ID: <20130723163806.GA27905@redhat.com> References: <1374573914.30532.68.camel@marge.simpson.net> <20130723155854.GA26211@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130723155854.GA26211@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 On 07/23, Oleg Nesterov wrote: > > On 07/23, Mike Galbraith wrote: > > > > I received a report that glibc:elf/pldd hangs occasionally, and indeed.. > > > > for i in `seq 1 1000`; do taskset -c 3 pldd $$ > /dev/null 2>&1; done > > > > ..will do so. Rummage..... > > > > ptrace(PTRACE_DETACH) returns -ESRCH when the trap hasn't happened yet, > > which happens because pldd doesn't wait() before ptrace(PTRACE_DETACH). > > > > pldd source: > > > [...snip...] > > > > Seems this usually works only because cycles expended between attach and > > detach is usually enough to let trap happen so tracee can set its state > > to TASK_TRACED as PTRACE_DETACH expects it to be. > > > > Is this expected behavior? > > Yes. PTRACE_ATTACH + PTRACE_DETACH is not correct without wait() in > between, this is expected. > > PTRACE_DETACH like (almost) any other ptrace request needs the stopped > tracee. Otherwise, say, ptrace_disable() or flush_ptrace_hw_breakpoint() > are not safe. I have found the source of pldd.c. It seems that it has another reason for waitpid(). /* Stop all threads since otherwise the list of loaded modules might change while we are reading it. */ Yes, but without waitpid() we can't know if it was actually stopped. OTOH, in this particular case pldd.c doesn't really need PTRACE_DETACH, it can simply exit. Oleg.