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Date: Thu, 20 Jul 2023 11:00:27 -0700
From: Jakub Kicinski <kuba@kernel.org>
To: Alexander Lobakin <aleksander.lobakin@intel.com>
Cc: "David S. Miller" <davem@davemloft.net>, Eric Dumazet
 <edumazet@google.com>, Paolo Abeni <pabeni@redhat.com>, Maciej Fijalkowski
 <maciej.fijalkowski@intel.com>, Larysa Zaremba <larysa.zaremba@intel.com>,
 Yunsheng Lin <linyunsheng@huawei.com>, Alexander Duyck
 <alexanderduyck@fb.com>, Jesper Dangaard Brouer <hawk@kernel.org>, "Ilias
 Apalodimas" <ilias.apalodimas@linaro.org>, <netdev@vger.kernel.org>,
 <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RFC net-next v2 7/7] net: skbuff: always try to recycle
 PP pages directly when in softirq
Message-ID: <20230720110027.4bd43ee7@kernel.org>
In-Reply-To: <8e65c3d3-c628-2176-2fc2-a1bc675ad607@intel.com>
References: <20230714170853.866018-1-aleksander.lobakin@intel.com>
	<20230714170853.866018-10-aleksander.lobakin@intel.com>
	<20230718174042.67c02449@kernel.org>
	<d7cd1903-de0e-0fe3-eb15-0146b589c7b0@intel.com>
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	<48c1d70b-d4bd-04c0-ab46-d04eaeaf4af0@intel.com>
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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 :)