netdev - Re: [PATCH net] net: dev: Detect dev_hold() after netdev_wait

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Date:   Thu, 27 Jan 2022 18:53:14 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Oliver Neukum <oneukum@...e.com>
Subject: Re: [PATCH net] net: dev: Detect dev_hold() after netdev_wait_allrefs()

On Thu, Jan 27, 2022 at 6:48 PM Jann Horn <jannh@...gle.com> wrote:

> When someone is using NET_DEV_REFCNT_TRACKER for slow debugging, they
> should also be able to take the performance hit of
> CONFIG_PCPU_DEV_REFCNT and rely on the normal increment-from-zero
> detection of the generic refcount code, right? (Maybe
> NET_DEV_REFCNT_TRACKER should depend on !CONFIG_PCPU_DEV_REFCNT?)

NET_DEV_REFCNT_TRACKER is not slow, I think it has neglectable cost really.
(I could not see any difference in my tests)

Also getting a trap at the exact moment of the buggy dev_hold_track()
is somewhat better than after-fact checking.

In your case, linkwatch_add_event() already uses dev_hold_track() so
my proposal would detect the issue right away.

>
> My intent with the extra check in free_netdev() was to get some
> limited detection for production systems that don't use
> NET_DEV_REFCNT_TRACKER.

Understood