linux-kernel - Re: [PATCH] specific do_timer

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu, 1 Dec 2011 14:56:23 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dimitri Sivanich <sivanich@....com>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] specific do_timer_cpu value for nohz off mode

On Thu, 1 Dec 2011 10:37:40 -0600
Dimitri Sivanich <sivanich@....com> wrote:

> +static ssize_t sysfs_store_do_timer_cpu(struct sys_device *dev,
> +						struct sysdev_attribute *attr,
> +						const char *buf, size_t size)
> +{
> +	struct sysdev_ext_attribute *ea = SYSDEV_TO_EXT_ATTR(attr);
> +	unsigned int new;
> +	int rv;
> +
> +#ifdef CONFIG_NO_HZ
> +	/* nohz mode not supported */
> +	if (tick_nohz_enabled)
> +		return -EINVAL;
> +#endif
> +
> +	rv = kstrtouint(buf, 0, &new);
> +	if (rv)
> +		return rv;
> +
> +	/* Protect against cpu-hotplug */
> +	get_online_cpus();
> +
> +	if (new >= nr_cpu_ids || !cpu_online(new)) {
> +		put_online_cpus();
> +		return -ERANGE;
> +	}
> +
> +	*(unsigned int *)(ea->var) = new;
> +
> +	put_online_cpus();
> +
> +	return size;
> +}

OK, I think this fixes one race.  We modify tick_do_timer_cpu inside
get_online_cpus().  If that cpu goes offline then
tick_handover_do_timer() will correctly hand the timer functions over
to a new CPU, and tick_handover_do_timer() runs in the CPU hotplug
handler which I assume is locked by get_online_cpus().  Please check
all this.

Now, the above code can alter tick_do_timer_cpu while a timer interrupt
is actually executing on another CPU.  Will this disrupt aything?  I
think it might cause problems.  If we take an interrupt on CPU 5 and
that CPU enters tick_periodic() and another CPU alters
tick_do_timer_cpu from 5 to 4 at exactly the correct time, tick_periodic()
might fail to run do_timer().  Or it might run do_timer() on both CPUs 4 and
5 concurrently?

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/