Re: [PATCH RFC net-next v2 7/7] net: skbuff: always try to recycle PP pages directly when in softirq

[Alexander Lobakin]

From: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
Date: Thu, 20 Jul 2023 20:01:28 +0200

> From: Jakub Kicinski <kuba@xxxxxxxxxx>
> Date: Thu, 20 Jul 2023 11:00:27 -0700
> 
>> On Thu, 20 Jul 2023 19:48:06 +0200 Alexander Lobakin wrote:
>>>>> My question was "how can two things race on one CPU in one context if it
>>>>> implies they won't ever happen simultaneously", but maybe my zero
>>>>> knowledge of netcons hides something from me.  
>>>>
>>>> One of them is in hardirq.  
>>>
>>> If I got your message correctly, that means softirq_count() can return
>>> `true` even if we're in hardirq context, but there are some softirqs
>>> pending? 
>>
>> Not pending, being executed. Hardirq can come during softirq.
>>
>>> I.e. if I call local_irq_save() inside NAPI poll loop,
>>> in_softirq() will still return `true`? (I'll check it myself in a bit,
>>> but why not ask).
>>
>> Yes.

My code, run from the NAPI poll loop:

	pr_info("BH. irq_count(): %lu", irq_count());
	pr_info("BH. in_hardirq(): %lu", in_hardirq());
	pr_info("BH. in_softirq(): %lu", in_softirq());
	pr_info("BH. in_serving_softirq(): %lu", in_serving_softirq());
	pr_info("BH. interrupt_context_level(): %u",
		interrupt_context_level());

	local_irq_save(flags);
	pr_info("TH. irq_count(): %lu", irq_count());
	pr_info("TH. in_hardirq(): %lu", in_hardirq());
	pr_info("TH. in_softirq(): %lu", in_softirq());
	pr_info("TH. in_serving_softirq(): %lu", in_serving_softirq());
	pr_info("TH. interrupt_context_level(): %u",
		interrupt_context_level());
	local_irq_restore(flags);

The result:

BH. irq_count(): 256
BH. in_hardirq(): 0
BH. in_softirq(): 256
BH. in_serving_softirq(): 256
BH. interrupt_context_level(): 1
TH. irq_count(): 256
TH. in_hardirq(): 0
TH. in_softirq(): 256
TH. in_serving_softirq(): 256
TH. interrupt_context_level(): 1

IOW, it reports we're in softirq no bloody matter if interrupts are
enabled or not. Either I did something wrong or the entire in_*irq()
family, including interrupt_context_level(), doesn't protect from
anything at all and doesn't work the way that most devs expect it to work?

(or was it just me? :D)

I guess the only way to be sure is to always check irqs_disabled() when
in_softirq() returns true.

>>
>>> Isn't checking for `interrupt_context_level() == 1` more appropriate
>>> then? Page Pool core code also uses in_softirq(), as well as a hellaton
>>> of other networking-related places.
>>
>> Right now page pool only supports BH and process contexts. IOW the
>> "else" branch of if (in_softirq()) in page pool is expecting to be
>> in process context.
>>
>> Supporting hard irq would mean we need to switch to _irqsave() locking.
>> That's likely way too costly.
>>
>> Or stash the freed pages away and free them lazily.
>>
>> Or add a lockdep warning and hope nobody will ever free a page-pool
>> backed skb from hard IRQ context :)
> 
> I told you under the previous version that this function is not supposed
> to be called under hardirq context, so we don't need to check for it :D
> But I was assuming nobody would try to do that. Seems like not really
> (netcons) if you want to sanitize this...
> 
> Thanks,
> Olek

Thanks,
Olek