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

From: Jakub Kicinski <[email-masked]>
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.

> 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 :)