Re: [PATCH -tip v3&10 07/18] x86: Add text_poke_smp for SMP cross modifying code

[Masami Hiramatsu <mhiramat@redhat.com>]

Mathieu Desnoyers wrote:
> * Masami Hiramatsu (mhiramat@redhat.com) wrote:
[...]
>> +
>> +/*
>> + * Cross-modifying kernel text with stop_machine().
>> + * This code originally comes from immediate value.
>> + */
>> +static atomic_t stop_machine_first;
>> +static int wrote_text;
>> +
>> +struct text_poke_params {
>> +	void *addr;
>> +	const void *opcode;
>> +	size_t len;
>> +};
>> +
>> +static int __kprobes stop_machine_text_poke(void *data)
>> +{
>> +	struct text_poke_params *tpp = data;
>> +
>> +	if (atomic_dec_and_test(&stop_machine_first)) {
>> +		text_poke(tpp->addr, tpp->opcode, tpp->len);
>> +		smp_wmb();	/* Make sure other cpus see that this has run */
>> +		wrote_text = 1;
>> +	} else {
>> +		while (!wrote_text)
>> +			smp_rmb();
>> +		sync_core();
>
> Hrm, there is a problem in there. The last loop, when wrote_text becomes
> true, does not perform any smp_mb(), so you end up in a situation where
> cpus in the "else" branch may never issue any memory barrier. I'd rather
> do:

Hmm, so how about this? :)
---
} else {
	do {
		smp_rmb();
	while (!wrote_text);
	sync_core();
}
---

>
> +static volatile int wrote_text;
>
> ...
>
> +static int __kprobes stop_machine_text_poke(void *data)
> +{
> +	struct text_poke_params *tpp = data;
> +
> +	if (atomic_dec_and_test(&stop_machine_first)) {
> +		text_poke(tpp->addr, tpp->opcode, tpp->len);
> +		smp_wmb();	/* order text_poke stores before store to wrote_text */
> +		wrote_text = 1;
> +	} else {
> +		while (!wrote_text)
> +			cpu_relax();
> +		smp_mb();	/* order wrote_text load before following execution */
> +	}
>
> If you don't like the "volatile int" definition of wrote_text, then we
> should probably use the ACCESS_ONCE() macro instead.

hm, yeah, volatile will be required.

Thank you,


-- 
Masami Hiramatsu
e-mail: mhiramat@redhat.com