Re: [PATCH tip 0/9] tracing: attach eBPF programs to tracepoints/syscalls/kprobe

[Alexei Starovoitov <ast-uqk4Ao+rVK5Wk0Htik3J/w@public.gmane.org>]

On Wed, Jan 21, 2015 at 5:56 PM, Steven Rostedt <rostedt-nx8X9YLhiw1AfugRpC6u6w@public.gmane.org> wrote:
> On Wed, 21 Jan 2015 17:49:08 -0800
> Alexei Starovoitov <ast-uqk4Ao+rVK5Wk0Htik3J/w@public.gmane.org> wrote:
>
>
>> > This also makes it keeping events in the soft-disabled state.
>>
>> I was never able to figure out the use case for soft-disabled state.
>> Probably historical before static_key was done.
>
> No, it's not historical at all. The "soft-disable" is a way to enable
> from any context. You can't enable a static key from NMI or interrupt
> context, but you can enable a "soft-disable" there.
>
> As you can enable or disable events from any function that the function
> tracer may trace, I needed a way to enable them (make the tracepoint
> active), but do nothing until something else turns them on.

Thanks for explanation. Makes sense.
Speaking of nmi... I think I will add a check that if (in_nmi())
just skip running the program, since supporting this use
case is not needed at the moment.