On Wed, 03 Oct 2012 18:23:09 +0530
"Srivatsa S. Bhat" <srivatsa.bhat@xxxxxxxxxxxxxxxxxx> 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@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);
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?
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.
--- 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) {
+ 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);
}
_
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