From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754489AbZFACqS (ORCPT ); Sun, 31 May 2009 22:46:18 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753724AbZFACqK (ORCPT ); Sun, 31 May 2009 22:46:10 -0400 Received: from cn.fujitsu.com ([222.73.24.84]:53575 "EHLO song.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1753549AbZFACqJ (ORCPT ); Sun, 31 May 2009 22:46:09 -0400 Message-ID: <4A23402B.1020601@cn.fujitsu.com> Date: Mon, 01 Jun 2009 10:42:51 +0800 From: Lai Jiangshan User-Agent: Thunderbird 2.0.0.6 (Windows/20070728) MIME-Version: 1.0 To: Andrew Morton CC: rusty@rustcorp.com.au, mingo@elte.hu, paulmck@linux.vnet.ibm.com, linux-kernel@vger.kernel.org, Oleg Nesterov , Linus Torvalds , Gautham R Shenoy Subject: Re: [PATCH 2/2] cpuhotplug: introduce try_get_online_cpus() References: <4A1F9CEE.5090305@cn.fujitsu.com> <20090529133118.1c7b16c2.akpm@linux-foundation.org> In-Reply-To: <20090529133118.1c7b16c2.akpm@linux-foundation.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Andrew Morton wrote: > >> --- >> diff --git a/include/linux/cpu.h b/include/linux/cpu.h >> index 2643d84..98f5c4b 100644 >> --- a/include/linux/cpu.h >> +++ b/include/linux/cpu.h >> @@ -104,6 +104,7 @@ extern struct sysdev_class cpu_sysdev_class; >> >> extern void get_online_cpus(void); >> extern void put_online_cpus(void); >> +extern int try_get_online_cpus(void); >> #define hotcpu_notifier(fn, pri) { \ >> static struct notifier_block fn##_nb __cpuinitdata = \ >> { .notifier_call = fn, .priority = pri }; \ >> @@ -117,6 +118,7 @@ int cpu_down(unsigned int cpu); >> >> #define get_online_cpus() do { } while (0) >> #define put_online_cpus() do { } while (0) >> +#define try_get_online_cpus() (1) >> #define hotcpu_notifier(fn, pri) do { (void)(fn); } while (0) >> /* These aren't inline functions due to a GCC bug. */ >> #define register_hotcpu_notifier(nb) ({ (void)(nb); 0; }) >> diff --git a/kernel/cpu.c b/kernel/cpu.c >> index 62198ec..e948f19 100644 >> --- a/kernel/cpu.c >> +++ b/kernel/cpu.c >> @@ -66,6 +66,15 @@ void put_online_cpus(void) >> } >> EXPORT_SYMBOL_GPL(put_online_cpus); >> >> +int try_get_online_cpus(void) >> +{ >> + might_sleep(); >> + if (cpu_hotplug.active_writer == current) >> + return 1; >> + return down_read_trylock(&cpu_hotplug.rwlock); >> + >> +} >> +EXPORT_SYMBOL_GPL(try_get_online_cpus); > > It's strange to add a might_sleep() to a function which doesn't sleep. It might sleep indeed. I prefer to add a might_sleep()/cond_sched() for a sleepable function. > > The patch adds no callers to this function. This is significant > because it would be quite interesting to find out which subsystem(s) > you've found to have this deadlock. I do think that we should look at > alternative (non-trylocky) ways of fixing them. > This problem exist in cgroup/cpuset. and Max Krasnyansky fix it: (non-trylocky way) * * The rebuild_sched_domains() and partition_sched_domains() * routines must nest cgroup_lock() inside get_online_cpus(), * but such cpuset changes as these must nest that locking the * other way, holding cgroup_lock() for much of the code. * * So in order to avoid an ABBA deadlock, the cpuset code handling * these user changes delegates the actual sched domain rebuilding * to a separate workqueue thread, which ends up processing the * above do_rebuild_sched_domains() function. */ static void async_rebuild_sched_domains(void) { queue_work(cpuset_wq, &rebuild_sched_domains_work); } get_online_cpus() is so a coarsely granular lock, and try_get_online_cpus() fails rarely. The kernel indeed needs try_get_online_cpus(). Paul will use it: http://lkml.org/lkml/2009/5/22/332 Lai