linux-kernel - Re: [PATCH v2 linux-trace 1/8] tracing: attach eBPF programs to tracepoints and syscalls

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Date:	Wed, 28 Jan 2015 22:39:34 -0800
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Namhyung Kim <namhyung@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 linux-trace 1/8] tracing: attach eBPF programs to
 tracepoints and syscalls

On Wed, Jan 28, 2015 at 9:39 PM, Masami Hiramatsu
<masami.hiramatsu.pt@...achi.com> wrote:
>
> Maybe, would we need a reference counter for each event? :)

when we would want multiple users to attach different programs
to the same event, then yes.
Right now I'd rather have things simple.

> Actually, ftrace event is not similar to perf-event which ktap
> is based on, ftrace event interface is always exported via
> debugfs, this means users can share the event for different
> usage.

yes.I've been thinking to extend perf_event ioctl to attach programs,
but right now it's only supporting tracepoints and kprobe
seems not trivial to add.
So I went for tracefs style of attaching for now.

> One possible other solution is to add a instance-lock interface
> for each ftrace instance and lock it by bpf. Then, other users
> can not enable/disable the events in the instance.

the user space can synchronize itself via flock. kernel doesn't
need to arbitrate. If one user process attached a program
that auto-enabled an event and another process did
'echo 0 > enable', it's fine. I think it's a feature instead of a bug.
Both users are root anyway.

The more we talk about it, the more I like a new 'bpf' file
approach within tracefs (that I've mentioned in cover letter)
with auto-enable/disable logic to make it clear that
it's different from traditional global 'filter' file.
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