netdev - Re: [RFC -next 1/2] lib: add reference counting infrastructure

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Message-ID: <20211117120347.5176b96f@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 17 Nov 2021 12:03:47 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [RFC -next 1/2] lib: add reference counting infrastructure

On Wed, 17 Nov 2021 11:20:30 -0800 Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> It can be hard to track where references are taken and released.
> 
> In networking, we have annoying issues at device dismantles,
> and we had various proposals to ease root causing them.
> 
> This patch adds new infrastructure pairing refcount increases
> and decreases. This will self document code, because programmer
> will have to associate increments/decrements.
> 
> This is controled by CONFIG_REF_TRACKER which can be selected
> by users of this feature.
> 
> This adds both cpu and memory costs, and thus should be reserved
> for debug kernel builds, or be enabled on demand with a static key.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Looks great, this is what I had in mind when I said:

| In the future we can extend this structure to also catch those
| who fail to release the ref on unregistering notification.

I realized today we can get quite a lot of coverage by just plugging 
in object debug infra.

The main differences I see:
 - do we ever want to use this in prod? - if not why allocate the
   tracker itself dynamically? The double pointer interface seems
   harder to compile out completely
 - whether one stored netdev ptr can hold multiple refs
 - do we want to wrap the pointer itself or have the "tracker" object
   be a separate entity
 - do we want to catch "use after free" when ref is accessed after
   it was already released

No strong preference either way.