linux-kernel - Re: [PATCH v1] usb: typec: tcpm: enforce ready state when queueing alt mode vdm

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Date: Thu, 25 Apr 2024 10:37:21 -0700
From: RD Babiera <rdbabiera@...gle.com>
To: Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Cc: linux@...ck-us.net, gregkh@...uxfoundation.org, hdegoede@...hat.com, 
	badhri@...gle.com, linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org, 
	stable@...r.kernel.org
Subject: Re: [PATCH v1] usb: typec: tcpm: enforce ready state when queueing
 alt mode vdm

On Thu, Apr 25, 2024 at 2:49 AM Heikki Krogerus
<heikki.krogerus@...ux.intel.com> wrote:
> I'm probable missing something, but wouldn't it be safer to check
> port->state after grabbing the lock?

I could have been more explicit in describing the deadlock, my bad.
But there are two
threads here:

Thread A starts in the TCPM. It is the port state machine that
transitions to DR_SWAP
and holds port->lock. When it unregisters DisplayPort Alt Mode, it goes into the
DP Alt Mode driver and hangs until any DP Alt Mode work is finished.

Thread B starts in the DP Alt Mode driver. It attempts to call tcpm_enter_mode
and the call to mutex_lock in tcpm_queue_vdm_unlock hangs because Thread A
holds the lock. Thread A will never drop the lock because it waits for Thread B
to finish.

So, the check is done before grabbing the lock because the thread needs to avoid
grabbing the lock in the first place. If port->state changes between
queueing and
sending the message, the VDM state machine will drop the message anyways
because port->state isn't in the ready state as well.

best,
rd