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Date: Thu, 2 May 2024 16:24:04 +0000
From: Sakari Ailus <sakari.ailus@linux.intel.com>
To: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
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Subject: Re: [PATCH 0/2] Introduce v4l2_async_nf_unregister_cleanup
Message-ID: <ZjO-JDBdHXVLbz5H@kekkonen.localdomain>
References: <20240502-master-v1-0-8bd109c6a3ba@collabora.com>
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 <20240502160830.GB11443@pendragon.ideasonboard.com>
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In-Reply-To: <20240502160830.GB11443@pendragon.ideasonboard.com>

Hi Laurent,

On Thu, May 02, 2024 at 07:08:30PM +0300, Laurent Pinchart wrote:
> On Thu, May 02, 2024 at 04:01:45PM +0000, Sakari Ailus wrote:
> > On Thu, May 02, 2024 at 06:56:26PM +0300, Laurent Pinchart wrote:
> > > On Thu, May 02, 2024 at 05:22:20PM +0200, Julien Massot wrote:
> > > > Many drivers has
> > > >   v4l2_async_nf_unregister(&notifier);
> > > >   v4l2_async_nf_cleanup(&notifier);
> > > > 
> > > > Introduce a helper function to call both functions in one line.
> > > 
> > > Does this really go in the right direction ? For other objects (video
> > > devices, media devices, ...), the unregistration should be done at
> > > .remove() time, and the cleanup at .release() time (the operation called
> > > when the last reference to the object is released). This is needed to
> > > ensure proper lifetime management of the objects, and avoid a
> > > use-after-free for objects that can be reached from userspace.
> > > 
> > > It could be argued that the notifier isn't exposed to userspace, but can
> > > we guarantee that no driver will have a need to access the notifier in a
> > > code path triggered by a userspace operation ? I think it would be safer
> > > to adopt the same split for the nofifier unregistration and cleanup. In
> > > my opinion using the same rule across different APIs also make it easier
> > > for driver authors and for reviewers to get it right.
> > > 
> > > As shown by your series, lots of drivers call v4l2_async_nf_cleanup()
> > > and .remove() time instead of .release(). That's because most drivers
> > > get lifetime management wrong and don't even implement .release().
> > > That's something Sakari is addressing with ongoing work. This patch
> > > series seems to go in the opposite direction.
> > 
> > This still avoids the driver authors feeling they need to implement wrapper
> > functions for v4l2_async_nf_{unregister,cleanup}. I'd be in favour merging
> > this.
> > 
> > I don't see this getting in the way of adding use counts as the code will
> > need to be changed in any case.
> 
> Fixing the lifetime issues would essentially revert 2/2 and move the
> v4l2_async_nf_cleanup() call to .remove(). I don't think providing a
> helper that forces the cleanup at .remove() time is a good idea, it
> gives a false sense of doing things right to drivers. This is the same
> reason why devm_kzalloc() is so harmful, it gave the wrong message, and
> created (or participated in) all those lifetime issues.

I still prefer having devm_*alloc() functions than having the drivers open
coding the same -- with the same result. The frameworks won't enable doing
this right at the moment and I don't think drivers (or us!) should be
penalised for that. The driver authors will only change what they do, with
these patches or without, when told so. But we don't really have an
alternative today.

A similar situation exists with clk_unprepare() and clk_disable().

-- 
Kind regards,

Sakari Ailus