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

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Message-ID: <20251017184415.GE316284@nvidia.com>
Date: Fri, 17 Oct 2025 15:44:15 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Tzung-Bi Shih <tzungbi@...nel.org>, Benson Leung <bleung@...omium.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J . Wysocki" <rafael@...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,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Bartosz Golaszewski <brgl@...ev.pl>,
	Wolfram Sang <wsa+renesas@...g-engineering.com>,
	Simona Vetter <simona.vetter@...ll.ch>,
	Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v5 5/7] revocable: Add fops replacement

On Fri, Oct 17, 2025 at 08:19:06PM +0200, Danilo Krummrich wrote:
> On 10/17/25 6:37 PM, Jason Gunthorpe wrote:
> > On Fri, Oct 17, 2025 at 06:29:10PM +0200, Danilo Krummrich wrote:
> > 
> >> I'm not sure about MISC device though. Unless there's a good reason,
> >> I think MISC device should be "fenced" instead.
> > 
> > misc is a very small wrapper around raw fops, and raw fops are
> > optimized for performance. Adding locking that many important things
> > like normal files don't need to all fops would not be agreed.
> > 
> > The sketch in this series where we have a core helper to provide a
> > shim fops that adds on the lock is smart and I think could be an
> > agreeable way to make a synchronous misc and cdev unregister for
> > everyone to trivially use.
> 
> Sure, for MISC devices without a parent for instance there are no device
> resources to access anyways.

There are many situations with misc that can get people into trouble without
parent:

 misc_deregister(x);
 timer_shutdown_sync(y);
 kfree(z);

For example. It is is buggy if the fops touch y or z.

This is why a _sync version is such a nice clean idea because with 5
letters the above can just be fixed.

Wrapping everything in a revocable would be a huge PITA.

Jason