From: RD Babiera <rdbabiera@google.com>
To: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Cc: linux@roeck-us.net, gregkh@linuxfoundation.org,
hdegoede@redhat.com, badhri@google.com,
linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
stable@vger.kernel.org
Subject: Re: [PATCH v1] usb: typec: tcpm: enforce ready state when queueing alt mode vdm
Date: Thu, 25 Apr 2024 10:37:21 -0700 [thread overview]
Message-ID: <CALzBnUHL4kWTQEncWfzNyze25_Ss_Pf8pyR2FOauyoLwNRrrMA@mail.gmail.com> (raw)
In-Reply-To: <ZionHYcUvvc865z4@kuha.fi.intel.com>
On Thu, Apr 25, 2024 at 2:49 AM Heikki Krogerus
<heikki.krogerus@linux.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
next prev parent reply other threads:[~2024-04-25 17:37 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-23 20:23 [PATCH v1] usb: typec: tcpm: enforce ready state when queueing alt mode vdm RD Babiera
2024-04-25 9:49 ` Heikki Krogerus
2024-04-25 17:37 ` RD Babiera [this message]
2024-04-29 8:18 ` Heikki Krogerus
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