Re: [PATCH] CPU hotplug, debug: Detect imbalance between get_online_cpus() and put_online_cpus()

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 10/04/2012 02:43 AM, Andrew Morton wrote:
> On Wed, 03 Oct 2012 18:23:09 +0530
> "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com> wrote:
> 
>> The synchronization between CPU hotplug readers and writers is achieved by
>> means of refcounting, safe-guarded by the cpu_hotplug.lock.
>>
>> get_online_cpus() increments the refcount, whereas put_online_cpus() decrements
>> it. If we ever hit an imbalance between the two, we end up compromising the
>> guarantees of the hotplug synchronization i.e, for example, an extra call to
>> put_online_cpus() can end up allowing a hotplug reader to execute concurrently with
>> a hotplug writer. So, add a BUG_ON() in put_online_cpus() to detect such cases
>> where the refcount can go negative.
>>
>> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
>> ---
>>
>>  kernel/cpu.c |    1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/kernel/cpu.c b/kernel/cpu.c
>> index f560598..00d29bc 100644
>> --- a/kernel/cpu.c
>> +++ b/kernel/cpu.c
>> @@ -80,6 +80,7 @@ void put_online_cpus(void)
>>  	if (cpu_hotplug.active_writer == current)
>>  		return;
>>  	mutex_lock(&cpu_hotplug.lock);
>> +	BUG_ON(cpu_hotplug.refcount == 0);
>>  	if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
>>  		wake_up_process(cpu_hotplug.active_writer);
>>  	mutex_unlock(&cpu_hotplug.lock);
> 
> I think calling BUG() here is a bit harsh.  We should only do that if
> there's a risk to proceeding: a risk of data loss, a reduced ability to
> analyse the underlying bug, etc.
> 
> But a cpu-hotplug locking imbalance is a really really really minor
> problem!  So how about we emit a warning then try to fix things up? 

That would be better indeed, thanks!

> This should increase the chance that the machine will keep running and
> so will increase the chance that a user will be able to report the bug
> to us.
>

Yep, sounds good.
 
> 
> --- a/kernel/cpu.c~cpu-hotplug-debug-detect-imbalance-between-get_online_cpus-and-put_online_cpus-fix
> +++ a/kernel/cpu.c
> @@ -80,9 +80,12 @@ void put_online_cpus(void)
>  	if (cpu_hotplug.active_writer == current)
>  		return;
>  	mutex_lock(&cpu_hotplug.lock);
> -	BUG_ON(cpu_hotplug.refcount == 0);
> -	if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
> -		wake_up_process(cpu_hotplug.active_writer);
> +	if (!--cpu_hotplug.refcount) {

This won't catch it. We'll enter this 'if' condition only when cpu_hotplug.refcount was
decremented to zero. We'll miss out the case when it went negative (which we intended to detect).

> +		if (WARN_ON(cpu_hotplug.refcount == -1))
> +			cpu_hotplug.refcount++;	/* try to fix things up */
> +		if (unlikely(cpu_hotplug.active_writer))
> +			wake_up_process(cpu_hotplug.active_writer);
> +	}
>  	mutex_unlock(&cpu_hotplug.lock);
> 
>  }

So how about something like below:

------------------------------------------------------>

From: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Subject: [PATCH] CPU hotplug, debug: Detect imbalance between get_online_cpus() and put_online_cpus()

The synchronization between CPU hotplug readers and writers is achieved by
means of refcounting, safe-guarded by the cpu_hotplug.lock.

get_online_cpus() increments the refcount, whereas put_online_cpus() decrements
it. If we ever hit an imbalance between the two, we end up compromising the
guarantees of the hotplug synchronization i.e, for example, an extra call to
put_online_cpus() can end up allowing a hotplug reader to execute concurrently with
a hotplug writer. So, add a WARN_ON() in put_online_cpus() to detect such cases
where the refcount can go negative, and also attempt to fix it up, so that we can
continue to run.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/cpu.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index f560598..42bd331 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -80,6 +80,10 @@ void put_online_cpus(void)
 	if (cpu_hotplug.active_writer == current)
 		return;
 	mutex_lock(&cpu_hotplug.lock);
+
+	if (WARN_ON(!cpu_hotplug.refcount))
+		cpu_hotplug.refcount++; /* try to fix things up */
+
 	if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
 		wake_up_process(cpu_hotplug.active_writer);
 	mutex_unlock(&cpu_hotplug.lock);


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