Re: [RFC][PATCH 3/7] kprobes: Remove kretprobe hash

[peterz@infradead.org]

On Fri, Aug 28, 2020 at 03:00:59AM +0900, Masami Hiramatsu wrote:
> On Thu, 27 Aug 2020 18:12:40 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > +static void invalidate_rp_inst(struct task_struct *t, struct kretprobe *rp)
> > +{
> > +	struct invl_rp_ipi iri = {
> > +		.task = t,
> > +		.rp = rp,
> > +		.done = false
> > +	};
> > +
> > +	for (;;) {
> > +		if (try_invoke_on_locked_down_task(t, __invalidate_rp_inst, rp))
> > +			return;
> > +
> > +		smp_call_function_single(task_cpu(t), __invalidate_rp_ipi, &iri, 1);
> > +		if (iri.done)
> > +			return;
> > +	}
> 
> Hmm, what about making a status place holder and point it from
> each instance to tell it is valid or not?
> 
> struct kretprobe_holder {
> 	atomic_t refcnt;
> 	struct kretprobe *rp;
> };
> 
> struct kretprobe {
> ...
> 	struct kretprobe_holder	*rph;	// allocate at register
> ...
> };
> 
> struct kretprobe_instance {
> ...
> 	struct kretprobe_holder	*rph; // free if refcnt == 0
> ...
> };
> 
> cleanup_rp_inst(struct kretprobe *rp)
> {
> 	rp->rph->rp = NULL;
> }
> 
> kretprobe_trampoline_handler()
> {
> ...
> 	rp = READ_ONCE(ri->rph-rp);
> 	if (likely(rp)) {
> 		// call rp->handler
> 	} else
> 		rcu_call(ri, free_rp_inst_rcu);
> ...
> }
> 
> free_rp_inst_rcu()
> {
> 	if (!atomic_dec_return(ri->rph->refcnt))
> 		kfree(ri->rph);
> 	kfree(ri);
> }
> 
> This increase kretprobe_instance a bit, but make things simpler.
> (and still keep lockless, atomic op is in the rcu callback).

Yes, much better.

Although I'd _love_ to get rid of rp->data_size, then we can simplify
all of this even more. I was thinking we could then have a single global
freelist thing and add some per-cpu cache to it (say 4-8 entries) to
avoid the worst contention.

And then make function-graph use this, instead of the other way around
:-)