linux-kernel - Re: [PATCH 01/51] CPU hotplug: Provide lockless versions of callback registration functions

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Date:	Mon, 10 Feb 2014 16:41:04 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	ego@...ux.vnet.ibm.com
CC:	Oleg Nesterov <oleg@...hat.com>, paulus@...ba.org,
	rusty@...tcorp.com.au, peterz@...radead.org, tglx@...utronix.de,
	akpm@...ux-foundation.org, mingo@...nel.org,
	paulmck@...ux.vnet.ibm.com, tj@...nel.org, walken@...gle.com,
	linux@....linux.org.uk, linux-kernel@...r.kernel.org,
	Toshi Kani <toshi.kani@...com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH 01/51] CPU hotplug: Provide lockless versions of	callback
 registration functions

On 02/10/2014 04:21 PM, Gautham R Shenoy wrote:
> On Mon, Feb 10, 2014 at 02:45:55PM +0530, Srivatsa S. Bhat wrote:
>> Hi Gautham,
>>
>> On 02/08/2014 12:41 AM, Gautham R Shenoy wrote:
>>> On Thu, Feb 06, 2014 at 07:41:03PM +0100, Oleg Nesterov wrote:
>>>> On 02/06, Srivatsa S. Bhat wrote:
>>>>>
>>>>> The following method of CPU hotplug callback registration is not safe
>>>>> due to the possibility of an ABBA deadlock involving the cpu_add_remove_lock
>>>>> and the cpu_hotplug.lock.
>>>>
[...]
>> get_online_cpus();  // acquire mutex; update counter; release mutex
>>
>> register_cpu_notifier(); // acquire cpu_add_remove_lock ...
>>
>> put_online_cpus();
>>
>>> If it hasn't, then the
>>> following lockdep annotations to cpu-hotplug locking should do the
>>> trick.
>>>
>>
>> This patch looks good to me. I have a couple of suggestions though..
>>
> 
> Thanks. I have incorporated the suggestions. Could you check if the
> following looks good ?
> 
> ---
>  Add lockdep annotations for get/put_online_cpus() and
>  cpu_hotplug_begin()/cpu_hotplug_end().
> 
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Srivatsa Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
> Signed-off-by: Gautham R. Shenoy <ego@...ux.vnet.ibm.com>
> ---
[...]
> +/* Lockdep annotations for get/put_online_cpus() and cpu_hotplug_begin/end() */
> +#define cpuhp_lock_acquire_read() lock_map_acquire_read(&cpu_hotplug.dep_map)
> +#define cpuhp_lock_acquire()      lock_map_acquire(&cpu_hotplug.dep_map)
> +#define cpuhp_lock_release()      lock_map_release(&cpu_hotplug.dep_map)
> +
>  void get_online_cpus(void)
>  {
>  	might_sleep();
>  	if (cpu_hotplug.active_writer == current)
>  		return;
> +	cpuhp_lock_acquire_read();
>  	mutex_lock(&cpu_hotplug.lock);
>  	cpu_hotplug.refcount++;
>  	mutex_unlock(&cpu_hotplug.lock);
> @@ -87,6 +101,7 @@ void put_online_cpus(void)
>  	if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
>  		wake_up_process(cpu_hotplug.active_writer);
>  	mutex_unlock(&cpu_hotplug.lock);
> +	cpuhp_lock_release();
> 
>  }
>  EXPORT_SYMBOL_GPL(put_online_cpus);
> @@ -117,6 +132,7 @@ void cpu_hotplug_begin(void)
>  {
>  	cpu_hotplug.active_writer = current;
> 
> +	cpuhp_lock_acquire();

Shouldn't we move this to _after_ the for-loop? Because, that's when the
hotplug writer is really in a state equivalent to exclusive access to the
hotplug lock... Else, we might fool lockdep into believing that the hotplug
writer has the lock for write, and at the same time several readers have
the lock for read as well.. no?

Sorry I didn't notice this earlier.

>  	for (;;) {
>  		mutex_lock(&cpu_hotplug.lock);
>  		if (likely(!cpu_hotplug.refcount))
> @@ -131,6 +147,7 @@ void cpu_hotplug_done(void)
>  {
>  	cpu_hotplug.active_writer = NULL;
>  	mutex_unlock(&cpu_hotplug.lock);
> +	cpuhp_lock_release();
>  }
> 
>  /*
> 

Regards,
Srivatsa S. Bhat

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