linux-kernel - Re: [PATCH 0/3] Revert "revocable: Revocable resource management"

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Message-ID: <20260129133440.GD2223369@nvidia.com>
Date: Thu, 29 Jan 2026 09:34:40 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: Bartosz Golaszewski <bartosz.golaszewski@....qualcomm.com>,
	Johan Hovold <johan@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Danilo Krummrich <dakr@...nel.org>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	Tzung-Bi Shih <tzungbi@...nel.org>,
	Linus Walleij <linusw@...nel.org>, Jonathan Corbet <corbet@....net>,
	Shuah Khan <shuah@...nel.org>,
	Wolfram Sang <wsa+renesas@...g-engineering.com>,
	Simona Vetter <simona.vetter@...ll.ch>,
	Dan Williams <dan.j.williams@...el.com>, linux-doc@...r.kernel.org,
	linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
	Bartosz Golaszewski <brgl@...nel.org>
Subject: Re: [PATCH 0/3] Revert "revocable: Revocable resource management"

On Thu, Jan 29, 2026 at 12:38:50PM +0200, Laurent Pinchart wrote:
> On Wed, Jan 28, 2026 at 09:23:22PM -0400, Jason Gunthorpe wrote:
> > On Thu, Jan 29, 2026 at 03:08:22AM +0200, Laurent Pinchart wrote:
> > > > The latter already have robust schemes to help the driver shutdown and
> > > > end the concurrent operations. ie cancel_work_sync(),
> > > > del_timer_sync(), free_irq(), and *notifier_unregister().
> > > 
> > > One a side note, devm_request_irq() is another of the devm_* helpers
> > > that cause race conditions, as interrupt handlers can run right after
> > > .remove() returns, which drivers will most likely not handle correctly.
> > 
> > Yes! You *cannot* intermix devm and non-devm approaches without
> > creating very subtle bugs exactly like this. If your subsystem does
> > not provide a "devm register" helper its drivers shouldn't use devm.
> 
> I'd relax that rule a bit. There are resources that drivers must never,
> ever access after .remove(), such as MMIO. Using devm_ioremap* should
> therefore be safe in all cases.

Yeah, probably, but I've seen driver using devm before & after
non-devm and it is just too hard to tell if things are going to
even work right.

To be fair the IRQ issue is always more involved. The subsystem should
provide a state after unregistration where the memory is still around
and the IRQ path into the subsystem becomes a NOP. The driver then
frees the IRQ, fences work and releases the driver memory.

It is hard to do this sequence with devm..

I think a lot of places manage without this because seeing interrupts
after unregister is probably a rare race condition in their HW.

> > But sure, it is all easy once you figure out how to give the fops shim
> > some place to store all this state since people would not agree to
> > make this a universal cost to all fops.
> 
> I didn't see any push back against Dan's proposal to store that
> information in struct cdev, did I miss something ? 

I also don't see an issue with that, especially if we can stack misc
on top of cdev to share the same logic.

I think if you take that idea and the other proposal to shim the fops
with ones that use the cdev data then we can see some
cdev_unregister_sync() primitive.

Jason