netdev - Re: [wpan-next v2 1/9] net: ieee802154: hwsim: Ensure proper channel selection at probe time

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Date:   Sun, 23 Jan 2022 15:34:14 -0500
From:   Alexander Aring <alex.aring@...il.com>
To:     Miquel Raynal <miquel.raynal@...tlin.com>
Cc:     Stefan Schmidt <stefan@...enfreihafen.org>,
        linux-wpan - ML <linux-wpan@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "open list:NETWORKING [GENERAL]" <netdev@...r.kernel.org>,
        Xue Liu <liuxuenetmail@...il.com>,
        Marcel Holtmann <marcel@...tmann.org>,
        Harry Morris <harrymorris12@...il.com>,
        David Girault <david.girault@...vo.com>,
        Romuald Despres <romuald.despres@...vo.com>,
        Frederic Blain <frederic.blain@...vo.com>,
        Nicolas Schodet <nico@...fr.eu.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [wpan-next v2 1/9] net: ieee802154: hwsim: Ensure proper channel
 selection at probe time

Hi,

On Thu, 20 Jan 2022 at 06:21, Miquel Raynal <miquel.raynal@...tlin.com> wrote:
>
> Drivers are expected to set the PHY current_channel and current_page
> according to their default state. The hwsim driver is advertising being
> configured on channel 13 by default but that is not reflected in its own
> internal pib structure. In order to ensure that this driver consider the
> current channel as being 13 internally, we can call hwsim_hw_channel()
> instead of creating an empty pib structure.
>
> We assume here that kvfree_rcu(NULL) is a valid call.
>
> Fixes: f25da51fdc38 ("ieee802154: hwsim: add replacement for fakelb")
> Signed-off-by: Miquel Raynal <miquel.raynal@...tlin.com>
> ---
>  drivers/net/ieee802154/mac802154_hwsim.c | 10 +---------
>  1 file changed, 1 insertion(+), 9 deletions(-)
>
> diff --git a/drivers/net/ieee802154/mac802154_hwsim.c b/drivers/net/ieee802154/mac802154_hwsim.c
> index 8caa61ec718f..795f8eb5387b 100644
> --- a/drivers/net/ieee802154/mac802154_hwsim.c
> +++ b/drivers/net/ieee802154/mac802154_hwsim.c
> @@ -732,7 +732,6 @@ static int hwsim_add_one(struct genl_info *info, struct device *dev,
>  {
>         struct ieee802154_hw *hw;
>         struct hwsim_phy *phy;
> -       struct hwsim_pib *pib;
>         int idx;
>         int err;
>
> @@ -780,13 +779,8 @@ static int hwsim_add_one(struct genl_info *info, struct device *dev,
>
>         /* hwsim phy channel 13 as default */
>         hw->phy->current_channel = 13;
> -       pib = kzalloc(sizeof(*pib), GFP_KERNEL);
> -       if (!pib) {
> -               err = -ENOMEM;
> -               goto err_pib;
> -       }
> +       hwsim_hw_channel(hw, hw->phy->current_page, hw->phy->current_channel);

Probably you saw it already; this will end in a
"suspicious_RCU_usage", that's because of an additional lock check in
hwsim_hw_channel() which checks if rtnl is held. However, in this
situation it's not necessary to hold the rtnl lock because we know the
phy is not being registered yet.

Either we change it to rcu_derefence() but then we would reduce the
check if rtnl lock is being held or just simply initial the default
pib->channel here to 13 which makes that whole patch a one line fix.

- Alex