linux-kernel - Re: [PATCH v2] net: make unregister netdev warning timeout configurable

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Date:   Wed, 24 Mar 2021 10:40:15 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     David Miller <davem@...emloft.net>,
        Leon Romanovsky <leon@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] net: make unregister netdev warning timeout configurable

On Tue, Mar 23, 2021 at 7:49 AM Dmitry Vyukov <dvyukov@...gle.com> wrote:
>
> netdev_wait_allrefs() issues a warning if refcount does not drop to 0
> after 10 seconds. While 10 second wait generally should not happen
> under normal workload in normal environment, it seems to fire falsely
> very often during fuzzing and/or in qemu emulation (~10x slower).
> At least it's not possible to understand if it's really a false
> positive or not. Automated testing generally bumps all timeouts
> to very high values to avoid flake failures.
> Add net.core.netdev_unregister_timeout_secs sysctl to make
> the timeout configurable for automated testing systems.
> Lowering the timeout may also be useful for e.g. manual bisection.
> The default value matches the current behavior.
>
> Signed-off-by: Dmitry Vyukov <dvyukov@...gle.com>
> Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=211877
> Cc: netdev@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
>
> ---
> Changes since v1:
>  - use sysctl instead of a config
> ---

>         },
> +       {
> +               .procname       = "netdev_unregister_timeout_secs",
> +               .data           = &netdev_unregister_timeout_secs,
> +               .maxlen         = sizeof(unsigned int),
> +               .mode           = 0644,
> +               .proc_handler   = proc_dointvec_minmax,
> +               .extra1         = SYSCTL_ZERO,
> +               .extra2         = &int_3600,
> +       },
>         { }
>  };
>

If we allow the sysctl to be 0, then we risk a flood of pr_emerg()
(one per jiffy ?)

If you really want the zero value, you need to change pr_emerg() to
pr_emerg_ratelimited()

Also, please base your patch on net-next, to avoid future merge conflicts
with my prior patch add2d7363107 "net: set initial device refcount to 1".