linux-kernel - Re: [PATCH 1/3] usb: ucsi: Add missing ppm

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Message-ID: <ZaeFnId6Ln3VUE5n@cae.in-ulm.de>
Date: Wed, 17 Jan 2024 08:45:32 +0100
From: "Christian A. Ehrhardt" <lk@...e.de>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
	linux-usb@...r.kernel.org, Dell.Client.Kernel@...l.com,
	Neil Armstrong <neil.armstrong@...aro.org>,
	Hans de Goede <hdegoede@...hat.com>,
	Jack Pham <quic_jackp@...cinc.com>,
	Fabrice Gasnier <fabrice.gasnier@...s.st.com>,
	Samuel Čavoj <samuel@...oj.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] usb: ucsi: Add missing ppm_lock

On Wed, Jan 17, 2024 at 06:44:40AM +0100, Greg Kroah-Hartman wrote:
> On Tue, Jan 16, 2024 at 11:40:39PM +0100, Christian A. Ehrhardt wrote:
> > Calling ->sync_write must be done while holding the PPM lock as the
> > mailbox logic does not support concurrent commands.
> > 
> > Thus protect the only call to ucsi_acknowledge_connector_change
> > with the PPM lock as it calls ->sync_write. All other calls to
> > ->sync_write already happen under the PPM lock.
> > 
> > Signed-off-by: Christian A. Ehrhardt <lk@...e.de>
> > ---
> > NOTE: This is not a theoretical issue. I've seen problems resulting
> > from the missing lock on real hardware.
> 
> What commit id does this fix?

It's hard to tell (due to rewrites, logic and API changes). After
digging a bit more I think it is at least a theoretical issues since
the introduction of partner tasks.

I'll wait a bit for additional feedback and fix this and other issues
noticed by your patch bot (sorry for those) in the next iteration.

> Should it be cc: stable?

Not sure. The race is triggered much more ofter after the quirk added
in patch 3/3, so this may not be a practical issue before that.
I'll add the tag in the next iteration, though.

   thanks   Christian