Re: [PATCH v9 2/3] watchdog: add watchdog_cpumask sysctl to assist nohz

Don Zickus
2015-04-22T15:19:44Z

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