linux-kernel - Re: [PATCH v5 5/7] revocable: Add fops replacement

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Message-ID: <20251211134306.GC28411@pendragon.ideasonboard.com>
Date: Thu, 11 Dec 2025 22:43:06 +0900
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Wolfram Sang <wsa+renesas@...g-engineering.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jason Gunthorpe <jgg@...dia.com>,
	Tzung-Bi Shih <tzungbi@...nel.org>,
	Benson Leung <bleung@...omium.org>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	Danilo Krummrich <dakr@...nel.org>,
	Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	chrome-platform@...ts.linux.dev, linux-kselftest@...r.kernel.org,
	Bartosz Golaszewski <brgl@...ev.pl>,
	Simona Vetter <simona.vetter@...ll.ch>,
	Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v5 5/7] revocable: Add fops replacement

On Thu, Dec 11, 2025 at 05:36:57PM +0900, Wolfram Sang wrote:
> 
> > > Isn't there even prototype code from Dan Williams?
> > > 
> > > "[PATCH 1/3] cdev: Finish the cdev api with queued mode support"
> > > 
> > > https://lkml.org/lkml/2021/1/20/997
> > 
> > I mentioned that in my LPC talk in 2022 :-) I think we should merge that
> > (or a rebased, possibly improved version of it). I've meant to try
> > plumbing that series in V4L2 but couldn't find the time so far.
> 
> Yes, you mentioned it in 2022 but maybe not everyone in this thread is
> right now aware of it ;) The patch above got changes requested. I talked
> to Dan very briefly about it at Maintainers Summit 2023 and he was also
> open (back then) to pick it up again.

After discussing with Tzung-Bi today after his presentation (thank you
Tzung-Bi for your time, it helped me understand the problem you're
facing better), I wonder if this series is fixing the issue in the right
place.

At the core of the problem is a devm_kzalloc() call to allocate
driver-specific data. That data structure is then referenced from a
cdev, which can dereference is after it gets freed. It seems that
reference-counting the data structure instead of using devm_kzalloc()
could be a better solution.

-- 
Regards,

Laurent Pinchart