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); -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>