netdev - Re: [PATCH net-next 5/6] igb: Do not free q_vector unless new one was allocated

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Date:   Mon, 7 Nov 2022 19:55:26 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Jacob Keller <jacob.e.keller@...el.com>
Cc:     Leon Romanovsky <leon@...nel.org>,
        "Ruhl, Michael J" <michael.j.ruhl@...el.com>,
        "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        "G, GurucharanX" <gurucharanx.g@...el.com>
Subject: Re: [PATCH net-next 5/6] igb: Do not free q_vector unless new one
 was allocated

On Mon, 7 Nov 2022 10:35:14 -0800 Jacob Keller wrote:
> > I understand the issue what you are trying to solve, I just don't
> > understand your RCU code. I would expect calls to rcu_dereference()
> > in order to get q_vector and rcu_assign_pointer() to clear
> > adapter->q_vector[v_idx], but igb has none.
> 
> the uses of kfree_rcu were introduced by 5536d2102a2d ("igb: Combine 
> q_vector and ring allocation into a single function")
> 
> The commit doesn't mention switching from kfree to kfree_rcu and I 
> suspect that the igb driver is not actually really using RCU semantics 
> properly.
> 
> The closest explanation is that the get_stats64 function might be 
> accessing the ring and thus needs the RCU grace period.. but I think 
> you're right in that we're missing the necessary RCU access macros.

Alright, expecting a follow up for this.