Re: [PATCH]kprobes fix bug when probed on task and isr functions

[Andrew Morton <akpm@osdl.org>]

Keshavamurthy Anil S <anil.s.keshavamurthy@intel.com> wrote:
>
> 	This patch fixes a race condition where in system used to hang
> or sometime crash within minutes when kprobes are inserted on 
> ISR routine and a task routine.

It's desirable that the patch descriptions tell us _how_ a bug was fixed,
as well as what the bug was.  It means that people don't have to ask
questions like:

>  void __kprobes lock_kprobes(void)
>  {
> +	unsigned long flags = 0;
> +
> +	local_irq_save(flags);
>  	spin_lock(&kprobe_lock);
>  	kprobe_cpu = smp_processor_id();
> + 	local_irq_restore(flags);
>  }

what is this change trying to do?  If a lock is taken from both process and
irq contexts then local IRQs must be disabled for the entire period when the
lock is held, not just for a little blip like this.  If IRQ-context code is
running this function then the code is deadlockable.

Now, probably there's deep magic happening here and I'm wrong.  If so then
please explain the code's magic via a comment patch so the question doesn't
arise again, thanks.