linux-kernel - Re: [PATCH 1/1] container_of: Document container_of

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Message-ID: <2024061702-vexingly-hypocrisy-d93d@gregkh>
Date: Mon, 17 Jun 2024 12:44:55 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Jason Gunthorpe <jgg@...dia.com>,
	Matthew Wilcox <willy@...radead.org>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	Hans Verkuil <hverkuil@...all.nl>
Subject: Re: [PATCH 1/1] container_of: Document container_of_const() is
 preferred

On Mon, Jun 17, 2024 at 01:08:25PM +0300, Sakari Ailus wrote:
> There is a warning in kerneldoc documentation of container_of() that
> constness of @ptr is lost. While this is a suggestion container_of_const()
> should be used instead, the vast majority of new code still uses
> container_of():
> 
> $ git diff v6.8 v6.9|grep container_of\(|wc -l
> 788
> $ git diff v6.8 v6.9|grep container_of_const|wc -l
> 11

That is because container_of_const is new, and you don't normally go
back and change things unless you have to.  Which is what I am starting
to do for some cases now in the driver core interactions, but generally
it's rare to need this.

Also note that container_of_const does not work in an inline function,
which is another reason people might not want to use it.

> Make an explicit recommendation to use container_of_const(), unless @ptr
> is const but its container isn't.
> 
> Signed-off-by: Sakari Ailus <sakari.ailus@...ux.intel.com>
> ---
>  include/linux/container_of.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/container_of.h b/include/linux/container_of.h
> index 713890c867be..7563015ff165 100644
> --- a/include/linux/container_of.h
> +++ b/include/linux/container_of.h
> @@ -13,7 +13,9 @@
>   * @type:	the type of the container struct this is embedded in.
>   * @member:	the name of the member within the struct.
>   *
> - * WARNING: any const qualifier of @ptr is lost.
> + * WARNING: any const qualifier of @ptr is lost. container_of() should only be
> + * used in cases where @ptr is const and its container is not and you know what
> + * you're doing. Otherwise always use container_of_const().

I know of no cases where a @ptr would be const yet the container would
not be, do you?  So why say that here?  That implies that it is a valid
thing to actually do.

I don't understand the goal here, do you want to just not have new
usages use container_of() at all?  Or are you trying to warn people of a
common problem that they make?  Having a const @ptr is not normal in the
kernel, so this should be ok.  If not, send patches to fix up those
users please.

thanks,

greg k-h