Re: [PATCH] mwifiex: Fix possible ABBA deadlock

[Doug Anderson]

Hi,

On Mon, Nov 29, 2021 at 4:47 PM Brian Norris <briannorris@xxxxxxxxxxxx> wrote:
>
> Quoting Jia-Ju Bai <baijiaju1990@xxxxxxxxx>:
>
>   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@xxxxxxxxxxxx>
> Reported-by: TOTE Robot <oslab@xxxxxxxxxxxxxxx>
> Link: https://lore.kernel.org/linux-wireless/0e495b14-efbb-e0da-37bd-af6bd677ee2c@xxxxxxxxx/
> Signed-off-by: Brian Norris <briannorris@xxxxxxxxxxxx>
> ---
>
> 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(-)

Though I'm by no means an expert on this code and I wrote the patch in
question a long time ago, this seems reasonable to me. Thanks for
fixing.

Reviewed-by: Douglas Anderson <dianders@xxxxxxxxxxxx>