Re: [RFC PATCH tip/master 2/3] kprobes: Allocate kretprobe instance if its free list is empty

[Ingo Molnar <mingo@kernel.org>]

* Masami Hiramatsu <mhiramat@kernel.org> wrote:

> On Thu, 30 Mar 2017 08:53:32 +0200
> Ingo Molnar <mingo@kernel.org> wrote:
> 
> > 
> > * Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > 
> > > > So this is something I missed while the original code was merged, but the concept 
> > > > looks a bit weird: why do we do any "allocation" while a handler is executing?
> > > > 
> > > > That's fundamentally fragile. What's the maximum number of parallel 
> > > > 'kretprobe_instance' required per kretprobe - one per CPU?
> > > 
> > > It depends on the place where we put the probe. If the probed function will be
> > > blocked (yield to other tasks), then we need a same number of threads on
> > > the system which can invoke the function. So, ultimately, it is same
> > > as function_graph tracer, we need it for each thread.
> > 
> > So then put it into task_struct (assuming there's no kretprobe-inside-kretprobe 
> > nesting allowed).
> 
> No, that is possible to put several kretprobes on same thread, e.g.
> the func1() is called from func2(), user can put kretprobes for each
> function at same time.
> So the possible solution is to allocate new return-stack for each task_struct,
> and that is what the function-graph tracer did.
> 
> Anyway, I'm considering to integrate kretprobe_instance with the ret_stack.
> It will increase memory usage for kretprobes, but can provide safer way
> to allocate kretprobe_instance.

Ok, that sounds good to me.

Thanks,

	Ingo