Subject: Re: [PATCH] mwifiex: Fix possible ABBA deadlock
To:     Brian Norris <briannorris@chromium.org>
Cc:     amitkarwar@gmail.com, ganapathi017@gmail.com,
        sharvari.harisangam@nxp.com, huxinming820@gmail.com,
        kvalo@codeaurora.org, David Miller <davem@davemloft.net>,
        kuba@kernel.org, linux-wireless@vger.kernel.org,
        netdev@vger.kernel.org,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Doug Anderson <dianders@chromium.org>
References: <0e495b14-efbb-e0da-37bd-af6bd677ee2c@gmail.com>
 <YaV0pllJ5p/EuUat@google.com>
From:   Jia-Ju Bai <baijiaju1990@gmail.com>
Message-ID: <9528b94a-7043-132f-c0f3-b45c5e551fa0@gmail.com>
Date:   Tue, 30 Nov 2021 10:18:10 +0800
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Hi Brian,

Thanks for your reply and explanation!
The patch looks good to me :)


Best wishes,
Jia-Ju Bai

On 2021/11/30 8:47, Brian Norris wrote:
> Quoting Jia-Ju Bai <baijiaju1990@gmail.com>:
>
>    mwifiex_dequeue_tx_packet()
>       spin_lock_bh(&priv->wmm.ra_list_spinlock); --> Line 1432 (Lock A)
>       mwifiex_send_addba()
>         spin_lock_bh(&priv->sta_list_spinlock); --> Line 608 (Lock B)
>
>    mwifiex_process_sta_tx_pause()
>       spin_lock_bh(&priv->sta_list_spinlock); --> Line 398 (Lock B)
>       mwifiex_update_ralist_tx_pause()
>         spin_lock_bh(&priv->wmm.ra_list_spinlock); --> Line 941 (Lock A)
>
> Similar report for mwifiex_process_uap_tx_pause().
>
> While the locking expectations in this driver are a bit unclear, the
> Fixed commit only intended to protect the sta_ptr, so we can drop the
> lock as soon as we're done with it.
>
> IIUC, this deadlock cannot actually happen, because command event
> processing (which calls mwifiex_process_sta_tx_pause()) is
> sequentialized with TX packet processing (e.g.,
> mwifiex_dequeue_tx_packet()) via the main loop (mwifiex_main_process()).
> But it's good not to leave this potential issue lurking.
>
> Fixes: ("f0f7c2275fb9 mwifiex: minor cleanups w/ sta_list_spinlock in cfg80211.c")
> Cc: Douglas Anderson <dianders@chromium.org>
> Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
> Link: https://lore.kernel.org/linux-wireless/0e495b14-efbb-e0da-37bd-af6bd677ee2c@gmail.com/
> Signed-off-by: Brian Norris <briannorris@chromium.org>
> ---
>
> On Tue, Nov 23, 2021 at 11:31:34AM +0800, Jia-Ju Bai wrote:
>> I am not quite sure whether these possible deadlocks are real and how to fix
>> them if they are real.
>> Any feedback would be appreciated, thanks :)
> I think these are at least theoretically real, and so we should take
> something like the $subject patch probably. But I don't believe we can
> actually hit this due to the main-loop structure of this driver.
>
> Anyway, see the surrounding patch.
>
> Thanks,
> Brian
>
>
>   drivers/net/wireless/marvell/mwifiex/sta_event.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/wireless/marvell/mwifiex/sta_event.c b/drivers/net/wireless/marvell/mwifiex/sta_event.c
> index 80e5d44bad9d..7d42c5d2dbf6 100644
> --- a/drivers/net/wireless/marvell/mwifiex/sta_event.c
> +++ b/drivers/net/wireless/marvell/mwifiex/sta_event.c
> @@ -365,10 +365,12 @@ static void mwifiex_process_uap_tx_pause(struct mwifiex_private *priv,
>   		sta_ptr = mwifiex_get_sta_entry(priv, tp->peermac);
>   		if (sta_ptr && sta_ptr->tx_pause != tp->tx_pause) {
>   			sta_ptr->tx_pause = tp->tx_pause;
> +			spin_unlock_bh(&priv->sta_list_spinlock);
>   			mwifiex_update_ralist_tx_pause(priv, tp->peermac,
>   						       tp->tx_pause);
> +		} else {
> +			spin_unlock_bh(&priv->sta_list_spinlock);
>   		}
> -		spin_unlock_bh(&priv->sta_list_spinlock);
>   	}
>   }
>   
> @@ -400,11 +402,13 @@ static void mwifiex_process_sta_tx_pause(struct mwifiex_private *priv,
>   			sta_ptr = mwifiex_get_sta_entry(priv, tp->peermac);
>   			if (sta_ptr && sta_ptr->tx_pause != tp->tx_pause) {
>   				sta_ptr->tx_pause = tp->tx_pause;
> +				spin_unlock_bh(&priv->sta_list_spinlock);
>   				mwifiex_update_ralist_tx_pause(priv,
>   							       tp->peermac,
>   							       tp->tx_pause);
> +			} else {
> +				spin_unlock_bh(&priv->sta_list_spinlock);
>   			}
> -			spin_unlock_bh(&priv->sta_list_spinlock);
>   		}
>   	}
>   }