From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ob0-f179.google.com (mail-ob0-f179.google.com [209.85.214.179]) (using TLSv1 with cipher ECDHE-RSA-RC4-SHA (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority" (not verified)) by ozlabs.org (Postfix) with ESMTPS id CD6E3B710D for ; Sat, 5 May 2012 11:48:51 +1000 (EST) Received: by mail-ob0-f179.google.com with SMTP id up19so4953274obb.38 for ; Fri, 04 May 2012 18:48:51 -0700 (PDT) Date: Fri, 4 May 2012 18:47:28 -0700 From: Anton Vorontsov To: Andrew Morton , Peter Zijlstra 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 In-Reply-To: <1335869133.13683.125.camel@twins> Cc: linaro-kernel@lists.linaro.org, Mike Frysinger , user-mode-linux-devel@lists.sourceforge.net, linux-sh@vger.kernel.org, Richard Weinberger , linuxppc-dev@lists.ozlabs.org, Oleg Nesterov , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Paul Mundt , John Stultz , patches@linaro.org, KOSAKI Motohiro , Russell King , uclinux-dist-devel@blackfin.uclinux.org, linux-arm-kernel@lists.infradead.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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