linux-kernel - [PATCH] CPU hotplug, debug: Detect imbalance between get_online_cpus() and put_online

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:	Wed, 03 Oct 2012 18:23:09 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Jiri Kosina <jkosina@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
CC:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	"Paul E. McKenney" <paul.mckenney@...aro.org>,
	Josh Triplett <josh@...htriplett.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH] CPU hotplug, debug: Detect imbalance between get_online_cpus()
 and put_online_cpus()

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@...ux.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 BUG_ON() in put_online_cpus() to detect such cases
where the refcount can go negative.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.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);


--
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/