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

[Alexander Lobakin <aleksander.lobakin@intel.com>]

From: Jakub Kicinski <kuba@kernel.org>
Date: Wed, 19 Jul 2023 13:51:50 -0700

> On Wed, 19 Jul 2023 18:34:46 +0200 Alexander Lobakin wrote:
>>> What if we got here from netpoll? napi budget was 0, so napi_safe is
>>> false, but in_softirq() can be true or false.  
>>
>> If we're on the same CPU where the NAPI would run and in the same
>> context, i.e. softirq, in which the NAPI would run, what is the problem?
>> If there really is a good one, I can handle it here.
> 
> #define SOFTIRQ_BITS		8
> #define SOFTIRQ_MASK		(__IRQ_MASK(SOFTIRQ_BITS) << SOFTIRQ_SHIFT)
> # define softirq_count()	(preempt_count() & SOFTIRQ_MASK)
> #define in_softirq()		(softirq_count())

I do remember those, don't worry :)

> 
> I don't know what else to add beyond that and the earlier explanation.

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.

> 
> AFAIK pages as allocated by page pool do not benefit from the usual
> KASAN / KMSAN checkers, so if we were to double-recycle a page once
> a day because of a netcons race - it's going to be a month long debug
> for those of us using Linux in production.

if (!test_bit(&napi->state, NPSVC))

? It would mean we're not netpolling.
Otherwise, if this still is not enough, I'do go back to my v1 approach
with having a NAPI flag, which would tell for sure we're good to go. I
got confused by your "wouldn't just checking for softirq be enough"! T.T
Joking :D

Thanks,
Olek