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

[Srivatsa S. Bhat]

On 10/03/2012 05:52 PM, Srivatsa S. Bhat wrote:
> On 10/03/2012 03:16 PM, Jiri Kosina wrote:
>> On Wed, 3 Oct 2012, Jiri Kosina wrote:
>>
>>> Good question. I believe it should be safe to drop slab_mutex earlier, as 
>>> cachep has already been unlinked. I am adding slab people and linux-mm to 
>>> CC (the whole thread on LKML can be found at 
>>> https://lkml.org/lkml/2012/10/2/296 for reference).
>>>
[...]
> 
> But, I'm also quite surprised that the put_online_cpus() code as it stands today
> doesn't have any checks for the refcount going negative. I believe that such a
> check would be valuable to help catch cases where we might end up inadvertently
> causing an imbalance between get_online_cpus() and put_online_cpus(). I'll post
> that as a separate patch.
> 


-----------------------------------


From: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
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 BUG_ON() in put_online_cpus() to detect such cases
where the refcount can go negative.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
---

 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);


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