Re: [BUG] usb: typec: ucsi: possible deadlock in ucsi_pr_swap() and ucsi_handle_connector_change()

[Jack Pham <quic_jackp@quicinc.com>]

Hi Heikki,

On Wed, Feb 09, 2022 at 04:30:31PM +0200, 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)?

We encountered a slightly different twist to this bug.  Instead of
deadlocking, we see that the dr_swap() / pr_swap() operations actually
jump out of the wait_for_completion_timeout() immediately, even before
any partner change occurs.  This is because the con->complete may
already have its done flag set to true from the first time
ucsi_handle_connector_change() runs, and is never reset after that.

In addition to the unlocking below, I think we need to also add
reinit_completion() calls at the start of ucsi_{pr,dr}_swap().

Thanks,
Jack

> From 2ad06425a3df7be656f8a5b3c202aab45554fd17 Mon Sep 17 00:00:00 2001
> From: Heikki Krogerus <heikki.krogerus@linux.intel.com>
> Date: Wed, 9 Feb 2022 17:27:19 +0300
> Subject: [PATCH] usb: typec: ucsi: Test fix
> 
> Interim.
> 
> Not-Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
> ---
>  drivers/usb/typec/ucsi/ucsi.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/usb/typec/ucsi/ucsi.c b/drivers/usb/typec/ucsi/ucsi.c
> index f0c2fa19f3e0f..225104beda8be 100644
> --- a/drivers/usb/typec/ucsi/ucsi.c
> +++ b/drivers/usb/typec/ucsi/ucsi.c
> @@ -956,14 +956,18 @@ static int ucsi_dr_swap(struct typec_port *port, enum typec_data_role role)
>  	if (ret < 0)
>  		goto out_unlock;
>  
> +	mutex_unlock(&con->lock);
> +
>  	if (!wait_for_completion_timeout(&con->complete,
>  					msecs_to_jiffies(UCSI_SWAP_TIMEOUT_MS)))
> -		ret = -ETIMEDOUT;
> +		return -ETIMEDOUT;
> +
> +	return 0;
>  
>  out_unlock:
>  	mutex_unlock(&con->lock);
>  
> -	return ret < 0 ? ret : 0;
> +	return ret;
>  }
>  
>  static int ucsi_pr_swap(struct typec_port *port, enum typec_role role)
> @@ -992,11 +996,13 @@ static int ucsi_pr_swap(struct typec_port *port, enum typec_role role)
>  	if (ret < 0)
>  		goto out_unlock;
>  
> +	mutex_unlock(&con->lock);
> +
>  	if (!wait_for_completion_timeout(&con->complete,
> -				msecs_to_jiffies(UCSI_SWAP_TIMEOUT_MS))) {
> -		ret = -ETIMEDOUT;
> -		goto out_unlock;
> -	}
> +				msecs_to_jiffies(UCSI_SWAP_TIMEOUT_MS)))
> +		return -ETIMEDOUT;
> +
> +	mutex_lock(&con->lock);
>  
>  	/* Something has gone wrong while swapping the role */
>  	if (UCSI_CONSTAT_PWR_OPMODE(con->status.flags) !=
> @@ -1372,6 +1378,7 @@ void ucsi_unregister(struct ucsi *ucsi)
>  	ucsi->ops->async_write(ucsi, UCSI_CONTROL, &cmd, sizeof(cmd));
>  
>  	for (i = 0; i < ucsi->cap.num_connectors; i++) {
> +		complete(&ucsi->connector[i].complete);
>  		cancel_work_sync(&ucsi->connector[i].work);
>  		ucsi_unregister_partner(&ucsi->connector[i]);
>  		ucsi_unregister_altmodes(&ucsi->connector[i],
> -- 
> 2.34.1
>