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:
>> 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();
>> }
>> ---
>>
> 
> The ordering we are looking for here are:
> 
> Write-side: smp_wmb() orders text_poke stores before store to wrote_text.
> 
> Read-side: order wrote_text load before subsequent execution of modified
>            instructions.
> 
> Here again, strictly speaking, wrote_text load is not ordered with respect to
> following instructions. So maybe it's fine on x86-TSO specifically, but I would
> not count on this kind of synchronization to work in the general case.
> 
> Given the very small expected performance impact of this code path, I would
> recomment using the more solid/generic alternative below. If there is really a
> gain to get by creating this weird wait loop with strange memory barrier
> semantics, fine, otherwise I'd be reluctant to accept your proposals as
> obviously correct.
> 
> If you really, really want to go down the route of proving the correctness of
> your memory barrier usage, I can recommend looking at the memory barrier formal
> verification framework I did as part of my thesis. But, really, in this case,
> the performance gain is just not there, so there is no point in spending time
> trying to prove this.

OK, that was my misunderstand. and cpu_relax() will be better for HT processors.
I'll update it according to your code below.

Thank you,

> 
> Thanks,
> 
> Mathieu
> 
>>>
>>> +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
>>
>>
>>
> 

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