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Date:   Thu, 20 Jul 2023 10:12:31 -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: <20230720101231.7a5ff6cd@kernel.org>
In-Reply-To: <48c1d70b-d4bd-04c0-ab46-d04eaeaf4af0@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>
        <20230719135150.4da2f0ff@kernel.org>
        <48c1d70b-d4bd-04c0-ab46-d04eaeaf4af0@intel.com>
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On Thu, 20 Jul 2023 18:46:02 +0200 Alexander Lobakin wrote:
> 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:  
>  [...]  
> >>
> >> 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.

One of them is in hardirq.

> > 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))

if you have to the right check is !in_hardirq()

> ? 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

I guess the problem I'm concerned about can already happen.
I'll send a lockdep annotation shortly.