From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751881AbZHXImJ (ORCPT ); Mon, 24 Aug 2009 04:42:09 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751471AbZHXImH (ORCPT ); Mon, 24 Aug 2009 04:42:07 -0400 Received: from mx1.redhat.com ([209.132.183.28]:41110 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751439AbZHXImG (ORCPT ); Mon, 24 Aug 2009 04:42:06 -0400 Date: Mon, 24 Aug 2009 10:38:26 +0200 From: Oleg Nesterov To: Hiroshi Shimamoto Cc: Roland McGrath , Andrew Morton , linux-kernel@vger.kernel.org Subject: Re: [PATCH] fix race copy_process() vs de_thread() Message-ID: <20090824083826.GA475@redhat.com> References: <4A9210A4.4010108@ct.jp.nec.com> <20090824061420.341A9414DF@magilla.sf.frob.com> <4A923403.6010201@ct.jp.nec.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4A923403.6010201@ct.jp.nec.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 08/24, Hiroshi Shimamoto wrote: > > The point is that de_thread() waits until other thread calls wake_up_process(). > In __exit_signal() when sig->count == 2, the thread calls wake_up_process(), > and then de_thread() will continue. However if another thread is during > copy_process(), the sig->count is incremented at copy_signal(). That makes > no wake_up_process(). Yes. Imho signal->count must die. But I never had time to kill it. It is not needed. For example, __exit_signal() can just check thread_group_leader() instead of atomic_dec_and_test(sig->count). As for this bug, I'd like to think a bit more. But how about the patch below? With this patch - copy_process() increments signal/live only when we know we start the new thread - if copy_process() fails, we just check CLONE_THREAD. If true - do nothing, the counters were not changed. If false - just release ->signal, counters must be 1. Oleg. --- a/kernel/fork.c +++ b/kernel/fork.c @@ -815,11 +815,8 @@ static int copy_signal(unsigned long clo { struct signal_struct *sig; - if (clone_flags & CLONE_THREAD) { - atomic_inc(¤t->signal->count); - atomic_inc(¤t->signal->live); + if (clone_flags & CLONE_THREAD) return 0; - } sig = kmem_cache_alloc(signal_cachep, GFP_KERNEL); tsk->signal = sig; @@ -877,16 +874,6 @@ void __cleanup_signal(struct signal_stru kmem_cache_free(signal_cachep, sig); } -static void cleanup_signal(struct task_struct *tsk) -{ - struct signal_struct *sig = tsk->signal; - - atomic_dec(&sig->live); - - if (atomic_dec_and_test(&sig->count)) - __cleanup_signal(sig); -} - static void copy_flags(unsigned long clone_flags, struct task_struct *p) { unsigned long new_flags = p->flags; @@ -1239,6 +1226,8 @@ static struct task_struct *copy_process( } if (clone_flags & CLONE_THREAD) { + atomic_inc(¤t->signal->count); + atomic_inc(¤t->signal->live); p->group_leader = current->group_leader; list_add_tail_rcu(&p->thread_group, &p->group_leader->thread_group); } @@ -1282,7 +1271,8 @@ bad_fork_cleanup_mm: if (p->mm) mmput(p->mm); bad_fork_cleanup_signal: - cleanup_signal(p); + if (!(clone_flags & CLONE_THREAD)) + __cleanup_signal(p); bad_fork_cleanup_sighand: __cleanup_sighand(p->sighand); bad_fork_cleanup_fs: