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

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Message-ID: <20150121205643.4d8a3516@gandalf.local.home>
Date:	Wed, 21 Jan 2015 20:56:43 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	Namhyung Kim <namhyung@...nel.org>, Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Brendan Gregg <brendan.d.gregg@...il.com>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH tip 0/9] tracing: attach eBPF programs to
 tracepoints/syscalls/kprobe

On Wed, 21 Jan 2015 17:49:08 -0800
Alexei Starovoitov <ast@...mgrid.com> 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.

-- Steve

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