From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S934363AbbDVPTo (ORCPT <rfc822;w@1wt.eu>);
	Wed, 22 Apr 2015 11:19:44 -0400
Received: from mx1.redhat.com ([209.132.183.28]:34445 "EHLO mx1.redhat.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1754976AbbDVPTj (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Wed, 22 Apr 2015 11:19:39 -0400
Date: Wed, 22 Apr 2015 11:18:23 -0400
From: Don Zickus <dzickus@redhat.com>
To: Ulrich Obergfell <uobergfe@redhat.com>
Cc: Chris Metcalf <cmetcalf@ezchip.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrew Jones <drjones@redhat.com>,
        chai wen <chaiw.fnst@cn.fujitsu.com>,
        Fabian Frederick <fabf@skynet.be>, Aaron Tomlin <atomlin@redhat.com>,
        Ben Zhang <benzh@chromium.org>, Christoph Lameter <cl@linux.com>,
        Gilad Ben-Yossef <gilad@benyossef.com>,
        Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
        Jonathan Corbet <corbet@lwn.net>, linux-doc@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v9 2/3] watchdog: add watchdog_cpumask sysctl to assist
 nohz
Message-ID: <20150422151823.GV98296@redhat.com>
References: <20150416152808.GA16270@lerouge>
 <1429295838-6328-1-git-send-email-cmetcalf@ezchip.com>
 <1429295838-6328-2-git-send-email-cmetcalf@ezchip.com>
 <1642563828.4126628.1429625220545.JavaMail.zimbra@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1642563828.4126628.1429625220545.JavaMail.zimbra@redhat.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Tue, Apr 21, 2015 at 10:07:00AM -0400, Ulrich Obergfell wrote:
> 
> Chris,
> 
> I think it would also be nice to check the plausibility of the user input.
> 
> +int proc_watchdog_cpumask(struct ctl_table *table, int write,
> +                          void __user *buffer, size_t *lenp, loff_t *ppos)
> +{
> +        int err;
> +
> +        mutex_lock(&watchdog_proc_mutex);
> +        err = proc_do_large_bitmap(table, write, buffer, lenp, ppos);
> +        if (!err && write) {
> +                /* Remove impossible cpus to keep sysctl output cleaner. */
> +                cpumask_and(watchdog_cpumask, watchdog_cpumask,
> +                            cpu_possible_mask);
> +
> +                if (watchdog_enabled && watchdog_thresh)
> +                        smpboot_update_cpumask_percpu_thread(&watchdog_threads,
> +                                                             watchdog_cpumask);
> +        }
> +        mutex_unlock(&watchdog_proc_mutex);
> +        return err;
> +}
> 
> I think the user should only be allowed to specify a mask that is a subset of
> tick_nohz_full_mask as only those CPUs don't have a watchdog thread by default.
> In other words, the user should not be able to interfere with housekeeping CPUs.

Hi Uli,

I am not sure that is necessary.  This was supposed to be a debugging
interface for nohz (and possibly other technologies).  I think restricting
it to just tick_nohz makes it difficult to try out new things or debug
certain problems.

Personally, I feel anyone who will use this sys interface will need to do so
at their own risk.


Cheers,
Don

> 
> For example, add a plausibility check like so:
> 
>   save watchdog_cpumask because proc_do_large_bitmap() is going to change it
> 
>   proc_do_large_bitmap()
> 
>   // return an error if the user-specified mask includes a housekeeping CPU
>   if (watchdog_cpumask and 'negated tick_nohz_full_mask') {
>       restore saved watchdog_cpumask
>       return -EINVAL
>   }
> 
> 
> Regards,
> 
> Uli