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
[off-list ref] 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.