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Date:   Fri, 28 Aug 2020 03:00:59 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org, Eddy_Wu@trendmicro.com,
        x86@kernel.org, davem@davemloft.net, rostedt@goodmis.org,
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        linux-arch@vger.kernel.org, cameron@moodycamel.com,
        oleg@redhat.com, will@kernel.org, paulmck@kernel.org
Subject: Re: [RFC][PATCH 3/7] kprobes: Remove kretprobe hash
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        <20200827161754.359432340@infradead.org>
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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).

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>