linux-kernel - Re: [BUG] staging: wfx: possible deadlock in wfx_conf_tx() and wfx_add

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Date:   Tue, 01 Feb 2022 12:56:01 +0100
From:   Jérôme Pouiller <jerome.pouiller@...abs.com>
To:     Jia-Ju Bai <baijiaju1990@...il.com>,
        Hillf Danton <hdanton@...a.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        linux-staging@...ts.linux.dev,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] staging: wfx: possible deadlock in wfx_conf_tx() and wfx_add_interface()

Hello,

On Tuesday 1 February 2022 12:33:03 CET Hillf Danton wrote:
> On Tue, 1 Feb 2022 15:09:34 +0800 Jia-Ju Bai wrote:
> > Hello,
> >
> > My static analysis tool reports a possible deadlock in the wfx driver in
> > Linux 5.16:
> >
> > wfx_conf_tx()
> >    mutex_lock(&wdev->conf_mutex); --> Line 225 (Lock A)
> >    wfx_update_pm()
> >      wait_for_completion_timeout(&wvif->set_pm_mode_complete, ...); -->
> > Line 3019 (Wait X)
> >
> > wfx_add_interface()
> >    mutex_lock(&wdev->conf_mutex); --> Line 737 (Lock A)
> >    complete(&wvif->set_pm_mode_complete); --> Line 758 (Wake X)
> >
> > When wfx_conf_tx() is executed, "Wait X" is performed by holding "Lock
> > A". If wfx_add_interface() is executed at this time, "Wake X" cannot be
> > performed to wake up "Wait X" in wfx_conf_tx(), because "Lock A" has
> > been already hold by wfx_conf_tx(), causing a possible deadlock.
> > I find that "Wait X" is performed with a timeout, to relieve the
> > possible deadlock; but I think this timeout can cause inefficient execution.
> >
> > I am not quite sure whether this possible problem is real and how to fix
> > it if it is real.
> > Any feedback would be appreciated, thanks :)
> >
> >
> > Best wishes,
> > Jia-Ju Bai
> 
> Hey Jia-Ju
> 
> Thank you for reporting it.
> 
> Given the init_completion() prior to complete() in wfx_add_interface(),
> no waiter is waken up by the complete(), so it has nothing to do with
> the waiter in the conf path.

Absolutely. The completion is done by wfx_hif_pm_mode_complete_indication()
(which is not behind a mutex).

> BTW if the unusual wfx init is a real use case then we can add a new helper.

Indeed, it could make the code better. I don't know if there would be other
users.


-- 
Jérôme Pouiller