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Date:   Thu, 10 Feb 2022 16:30:23 +0200
From:   Heikki Krogerus <heikki.krogerus@linux.intel.com>
To:     Jia-Ju Bai <baijiaju1990@gmail.com>
Cc:     Greg KH <gregkh@linuxfoundation.org>, kyletso@google.com,
        jackp@codeaurora.org, andy.shevchenko@gmail.com,
        unixbhaskar@gmail.com, subbaram@codeaurora.org,
        mrana@codeaurora.org,
        "linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [BUG] usb: typec: ucsi: possible deadlock in ucsi_pr_swap() and
 ucsi_handle_connector_change()
Message-ID: <YgUhf9o5F98dZyxn@kuha.fi.intel.com>
References: <037de7ac-e210-bdf5-ec7a-8c0c88a0be20@gmail.com>
 <YgPQB9BYJcDzbd02@kuha.fi.intel.com>
 <43d6c3b5-17e6-63a8-21fa-3ff9f478ada7@gmail.com>
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On Thu, Feb 10, 2022 at 10:24:42AM +0800, Jia-Ju Bai wrote:
> 
> 
> On 2022/2/9 22:30, Heikki Krogerus wrote:
> > On Wed, Feb 09, 2022 at 11:50:57AM +0800, Jia-Ju Bai wrote:
> > > Hello,
> > > 
> > > My static analysis tool reports a possible deadlock in the ucsi driver in
> > > Linux 5.16:
> > > 
> > > ucsi_pr_swap()
> > >  ? mutex_lock(&con->lock); --> Line 962 (Lock A)
> > >  ? wait_for_completion_timeout(&con->complete, ...) --> Line 981 (Wait X)
> > > 
> > > ucsi_handle_connector_change()
> > >  ? mutex_lock(&con->lock); --> Line 763 (Lock A)
> > >  ? complete(&con->complete); --> Line 782 (Wake X)
> > >  ? complete(&con->complete); --> Line 807 (Wake X)
> > > 
> > > When ucsi_pr_swap() is executed, "Wait X" is performed by holding "Lock A".
> > > If ucsi_handle_connector_change() is executed at this time, "Wake X" cannot
> > > be performed to wake up "Wait X" in ucsi_handle_connector_change(), because
> > > "Lock A" has been already held by ucsi_handle_connector_change(), 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.
> > > Any feedback would be appreciated, thanks :)
> > This is probable a regression from commit ad74b8649bea ("usb: typec:
> > ucsi: Preliminary support for alternate modes"). Can you test does
> > this patch fix the issue (attached)?
> 
> Hi Heikki,
> 
> Thanks for the reply and patch.
> After the patch is used, my tool does not report this deadlock in the ucsi
> driver.
> Thus, I think this patch should be okay to fix the deadlock :)

Thanks for testing it. I'll tag you as the reporter and tester in the
patch.

Br,

-- 
heikki