From 075663d19885eb3738fd2d7dbdb8947e12563b68 Mon Sep 17 00:00:00 2001 From: "Srivatsa S. Bhat" Date: Mon, 8 Oct 2012 16:28:20 -0700 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, safeguarded 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 Reviewed-by: Yasuaki Ishimatsu Cc: Jiri Kosina Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Peter Zijlstra Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/cpu.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/kernel/cpu.c b/kernel/cpu.c index f560598807c1..42bd331ee0ab 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);