netdev - Re: [PATCH 04/45] CPU hotplug: Add infrastructure to check lacking hotplug synchronization

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Date:	Mon, 24 Jun 2013 19:26:03 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	tglx@...utronix.de, peterz@...radead.org, tj@...nel.org,
	oleg@...hat.com, paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	walken@...gle.com, vincent.guittot@...aro.org,
	laijs@...fujitsu.com, wangyun@...ux.vnet.ibm.com,
	xiaoguangrong@...ux.vnet.ibm.com, sbw@....edu, fweisbec@...il.com,
	zhong@...ux.vnet.ibm.com, nikunj@...ux.vnet.ibm.com,
	linux-pm@...r.kernel.org, linux-arch@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, Alex Shi <alex.shi@...el.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH 04/45] CPU hotplug: Add infrastructure to check lacking
 hotplug synchronization

On Sun, 2013-06-23 at 19:08 +0530, Srivatsa S. Bhat wrote:


Just to make the code a little cleaner, can you add:

> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 860f51a..e90d9d7 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -63,6 +63,72 @@ static struct {
>  	.refcount = 0,
>  };
>  
> +#ifdef CONFIG_DEBUG_HOTPLUG_CPU
> +
> +static DEFINE_PER_CPU(unsigned long, atomic_reader_refcnt);
> +
> +static int current_is_hotplug_safe(const struct cpumask *mask)
> +{
> +
> +	/* If we are not dealing with cpu_online_mask, don't complain. */
> +	if (mask != cpu_online_mask)
> +		return 1;
> +
> +	/* If this is the task doing hotplug, don't complain. */
> +	if (unlikely(current == cpu_hotplug.active_writer))
> +		return 1;
> +
> +	/* If we are in early boot, don't complain. */
> +	if (system_state != SYSTEM_RUNNING)
> +		return 1;
> +
> +	/*
> +	 * Check if the current task is in atomic context and it has
> +	 * invoked get_online_cpus_atomic() to synchronize with
> +	 * CPU Hotplug.
> +	 */
> +	if (preempt_count() || irqs_disabled())
> +		return this_cpu_read(atomic_reader_refcnt);
> +	else
> +		return 1; /* No checks for non-atomic contexts for now */
> +}
> +
> +static inline void warn_hotplug_unsafe(void)
> +{
> +	WARN_ONCE(1, "Must use get/put_online_cpus_atomic() to synchronize"
> +		     " with CPU hotplug\n");
> +}
> +
> +/*
> + * Check if the task (executing in atomic context) has the required protection
> + * against CPU hotplug, while accessing the specified cpumask.
> + */
> +void check_hotplug_safe_cpumask(const struct cpumask *mask)
> +{
> +	if (!current_is_hotplug_safe(mask))
> +		warn_hotplug_unsafe();
> +}
> +EXPORT_SYMBOL_GPL(check_hotplug_safe_cpumask);
> +
> +/*
> + * Similar to check_hotplug_safe_cpumask(), except that we don't complain
> + * if the task (executing in atomic context) is testing whether the CPU it
> + * is executing on is online or not.
> + *
> + * (A task executing with preemption disabled on a CPU, automatically prevents
> + *  offlining that CPU, irrespective of the actual implementation of CPU
> + *  offline. So we don't enforce holding of get_online_cpus_atomic() for that
> + *  case).
> + */
> +void check_hotplug_safe_cpu(unsigned int cpu, const struct cpumask *mask)
> +{
> +	if(!current_is_hotplug_safe(mask) && cpu != smp_processor_id())
> +		warn_hotplug_unsafe();
> +}
> +EXPORT_SYMBOL_GPL(check_hotplug_safe_cpu);
> +

static inline void atomic_reader_refcnt_inc(void)
{
	this_cpu_inc(atomic_reader_refcnt);
}
static inline void atomic_reader_refcnt_dec(void)
{
	this_cpu_dec(atomic_reader_refcnt);
}

#else
static inline void atomic_reader_refcnt_inc(void)
{
}
static inline void atomic_reader_refcnt_dec(void)
{
}
#endif

> +#endif
> +
>  void get_online_cpus(void)
>  {
>  	might_sleep();
> @@ -189,13 +255,22 @@ unsigned int get_online_cpus_atomic(void)
>  	 * from going offline.
>  	 */
>  	preempt_disable();
> +
> +#ifdef CONFIG_DEBUG_HOTPLUG_CPU
> +	this_cpu_inc(atomic_reader_refcnt);
> +#endif

Replace the #ifdef with just:

	atomic_reader_refcnt_inc();

>  	return smp_processor_id();
>  }
>  EXPORT_SYMBOL_GPL(get_online_cpus_atomic);
>  
>  void put_online_cpus_atomic(void)
>  {
> +
> +#ifdef CONFIG_DEBUG_HOTPLUG_CPU
> +	this_cpu_dec(atomic_reader_refcnt);
> +#endif

And

	atomic_reader_refcnt_dec();

-- Steve

>  	preempt_enable();
> +
>  }
>  EXPORT_SYMBOL_GPL(put_online_cpus_atomic);
>  


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