From mboxrd@z Thu Jan  1 00:00:00 1970
From: Alexei Starovoitov <ast-uqk4Ao+rVK5Wk0Htik3J/w@public.gmane.org>
Subject: Re: [PATCH tip 0/9] tracing: attach eBPF programs to tracepoints/syscalls/kprobe
Date: Wed, 21 Jan 2015 18:13:23 -0800
Message-ID: <CAMEtUuz9bcaYN0if1w1O01vwc593P2ACUX-KK++6Jeqz7MwFsg@mail.gmail.com>
References: <CAMEtUux8v2LDtLcgpT9hCvJgnrCwT2fkzsSvAPFSuEUx+itxyQ@mail.gmail.com>
 <20150121205643.4d8a3516@gandalf.local.home>
Mime-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Cc: Namhyung Kim <namhyung-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Ingo Molnar <mingo-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
	Arnaldo Carvalho de Melo <acme-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>,
	Jiri Olsa <jolsa-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	"David S. Miller" <davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org>,
	Daniel Borkmann <dborkman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	Hannes Frederic Sowa <hannes-tFNcAqjVMyqKXQKiL6tip0B+6BGkLq7r@public.gmane.org>,
	Brendan Gregg <brendan.d.gregg-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
	Linux API <linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	Network Development <netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	LKML <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
To: Steven Rostedt <rostedt-nx8X9YLhiw1AfugRpC6u6w@public.gmane.org>
Return-path: <linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
In-Reply-To: <20150121205643.4d8a3516-f9ZlEuEWxVcJvu8Pb33WZ0EMvNT87kid@public.gmane.org>
Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
List-Id: netdev.vger.kernel.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.