From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752844Ab0F1Lxe (ORCPT ); Mon, 28 Jun 2010 07:53:34 -0400 Received: from casper.infradead.org ([85.118.1.10]:36730 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751712Ab0F1Lxd convert rfc822-to-8bit (ORCPT ); Mon, 28 Jun 2010 07:53:33 -0400 Subject: [PATCH] init: Fix race between init and kthreadd -v2 From: Peter Zijlstra To: Ilya Loginov Cc: torvalds@linux-foundation.org, linux-kernel@vger.kernel.org, Ingo Molnar In-Reply-To: <1277715689.1875.1104.camel@laptop> References: <20100624001148.61e9da1c.isloginov@gmail.com> <1277385096.1875.974.camel@laptop> <20100624172334.ca7e9bef.isloginov@gmail.com> <1277715689.1875.1104.camel@laptop> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Mon, 28 Jun 2010 13:53:17 +0200 Message-ID: <1277725997.3561.6.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2010-06-28 at 11:01 +0200, Peter Zijlstra wrote: > static int __init kernel_init(void * unused) > { > + /* > + * Synchronize against setting kthreadd_task in rest_init(). > + * Using a mutex would have been a lot nicer, but since its a very > + * rare race don't bother wasting the space overhead. > + */ > + while (!kthreadd_task) > + yield(); > + > lock_kernel(); I just realized its all __init code so its all 'free' anyway, how about the nicer version: --- Subject: init: Fix race between init and kthreadd -v2 Ilya reported that on a very slow machine he could reliably reproduce a race between forking init and kthreadd. We first fork init so that it obtains pid-1, however since the scheduler is already fully running at this point it can preempt and run the init thread before we spawn and set kthreadd_task. The init thread can then attempt spawning kthreads without kthreadd being present which results in an OOPS. Signed-off-by: Peter Zijlstra --- init/main.c | 16 ++++++++++++++++ 1 files changed, 16 insertions(+), 0 deletions(-) diff --git a/init/main.c b/init/main.c index e2a2bf3..8f2acf5 100644 --- a/init/main.c +++ b/init/main.c @@ -420,18 +420,27 @@ static void __init setup_command_line(char *command_line) * gcc-3.4 accidentally inlines this function, so use noinline. */ +static __initdata DEFINE_MUTEX(kthreadd_lock); + static noinline void __init_refok rest_init(void) __releases(kernel_lock) { int pid; rcu_scheduler_starting(); + /* + * We need to spawn init first so that it obtains pid-1, however + * the init task will end up wanting to create kthreads, which + * if we schedule it before we create kthreadd, will OOPS. + */ + mutex_lock(&kthreadd_lock); kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND); numa_default_policy(); pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES); rcu_read_lock(); kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns); rcu_read_unlock(); + mutex_unlock(&kthreadd_lock); unlock_kernel(); /* @@ -847,6 +856,13 @@ static noinline int init_post(void) static int __init kernel_init(void * unused) { + /* + * We spawned this thread while holding this lock, ensure the + * locked section in rest_init() is complete before proceeding. + */ + mutex_lock(&kthreadd_lock); + mutex_unlock(&kthreadd_lock); + lock_kernel(); /*