From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx203.postini.com [74.125.245.203]) by kanga.kvack.org (Postfix) with SMTP id DAD376B0102 for ; Fri, 4 May 2012 21:48:51 -0400 (EDT) Received: by mail-ob0-f169.google.com with SMTP id wd18so6351090obb.14 for ; Fri, 04 May 2012 18:48:51 -0700 (PDT) Date: Fri, 4 May 2012 18:47:28 -0700 From: Anton Vorontsov Subject: [PATCH] cpu: Document clear_tasks_mm_cpumask() Message-ID: <20120505014728.GA9079@lizard> References: <20120423070641.GA27702@lizard> <20120423070736.GA30752@lizard> <20120426165911.00cebd31.akpm@linux-foundation.org> <1335869133.13683.125.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <1335869133.13683.125.camel@twins> Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton , Peter Zijlstra Cc: Oleg Nesterov , Russell King , Mike Frysinger , Benjamin Herrenschmidt , Richard Weinberger , Paul Mundt , KOSAKI Motohiro , John Stultz , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, uclinux-dist-devel@blackfin.uclinux.org, linuxppc-dev@lists.ozlabs.org, linux-sh@vger.kernel.org, user-mode-linux-devel@lists.sourceforge.net, linaro-kernel@lists.linaro.org, patches@linaro.org, linux-mm@kvack.org This patch adds more comments on clear_tasks_mm_cpumask, plus adds a runtime check: the function is only suitable for offlined CPUs, and if called inappropriately, the kernel should scream aloud. Suggested-by: Andrew Morton Suggested-by: Peter Zijlstra Signed-off-by: Anton Vorontsov --- On Tue, May 01, 2012 at 12:45:33PM +0200, Peter Zijlstra wrote: > On Thu, 2012-04-26 at 16:59 -0700, Andrew Morton wrote: > > > +void clear_tasks_mm_cpumask(int cpu) > > > > The operation of this function was presumably obvious to you at the > > time you wrote it, but that isn't true of other people at later times. > > > > Please document it? [...] > > If someone tries to use this function for a different purpose, or > > copies-and-modifies it for a different purpose, we just shot them in > > the foot. > > > > They'd be pretty dumb to do that without reading the local comment, > > but still... > > Methinks something simple like: > > WARN_ON(cpu_online(cpu)); > > Ought to cure that worry, no? :-) Yeah, this is all good ideas, thanks. How about the following patch? kernel/cpu.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/kernel/cpu.c b/kernel/cpu.c index ecdf499..1bfa26f 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -13,6 +13,7 @@ #include #include #include +#include #include #include #include @@ -173,6 +174,18 @@ void __ref unregister_cpu_notifier(struct notifier_block *nb) } EXPORT_SYMBOL(unregister_cpu_notifier); +/** + * clear_tasks_mm_cpumask - Safely clear tasks' mm_cpumask for a CPU + * @cpu: a CPU id + * + * This function walks up all processes, finds a valid mm struct for + * each one and then clears a corresponding bit in mm's cpumask. While + * this all sounds trivial, there are various non-obvious corner cases, + * which this function tries to solve in a safe manner. + * + * Also note that the function uses a somewhat relaxed locking scheme, + * so it may be called only for an already offlined CPU. + */ void clear_tasks_mm_cpumask(int cpu) { struct task_struct *p; @@ -184,10 +197,15 @@ void clear_tasks_mm_cpumask(int cpu) * Thus, we may use rcu_read_lock() here, instead of grabbing * full-fledged tasklist_lock. */ + WARN_ON(cpu_online(cpu)); rcu_read_lock(); for_each_process(p) { struct task_struct *t; + /* + * Main thread might exit, but other threads may still have + * a valid mm. Find one. + */ t = find_lock_task_mm(p); if (!t) continue; -- 1.7.9.2 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org