linux-kernel - Re: [PATCH] driver core: bus: Mark an impossible error path with WARN_ON() in bus_add

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Message-ID: <2024091510-scarcity-roman-4776@gregkh>
Date: Sun, 15 Sep 2024 16:57:05 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Zijun Hu <zijun_hu@...oud.com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, linux-kernel@...r.kernel.org,
	Zijun Hu <quic_zijuhu@...cinc.com>
Subject: Re: [PATCH] driver core: bus: Mark an impossible error path with
 WARN_ON() in bus_add_driver()

On Sun, Sep 15, 2024 at 10:36:44PM +0800, Zijun Hu wrote:
> On 2024/9/15 21:55, Greg Kroah-Hartman wrote:
> > On Sun, Sep 15, 2024 at 09:38:15PM +0800, Zijun Hu wrote:
> >> On 2024/9/15 21:00, Greg Kroah-Hartman wrote:
> >>> On Sun, Sep 15, 2024 at 06:22:05PM +0800, Zijun Hu wrote:
> >>>> From: Zijun Hu <quic_zijuhu@...cinc.com>
> >>>>
> >>>> driver_attach() called by bus_add_driver() always returns 0, so its
> >>>> corresponding error path will never happen, hence mark the impossible
> >>>> error path with WARN_ON() to remind readers to disregard it.
> >>>
> >>> So you just caused the machine to crash and reboot if that happens
> >>> (remember, panic-on-warn is enabled in a few billion Linux systems...)
> >>>
> >> are there good way to mark a if condition which is always or mostly
> >> evaluated to false currently without any side effect?
> > 
> > If always, then remove the code involved.  If mostly, just do it
> > normally.
> > 
> >> i think this is a generic requirement since readers may not want to
> >> care about things which will never or rarely happen, below link
> >> involves such discussion:
> >> https://lore.kernel.org/all/2024090444-earmark-showpiece-b3dc@gregkh/
> > 
> > Yes, but likely/unlikely is for performance, not for documentation.
> 
> if you git grep unlikely in current kernel tree, you will find
> that there are too many unlikely usages which should be irrelevant
> performance. you maybe look at drivers/base/devres.c.

Yes, and they should be removed, we have someone that normally sweeps
the tree every few years and runs a tool that can compare if those
actually are correct or not, and usually not.

> so i think one of purpose of unlikely may be for the requirement i
> mentioned.

Nope, sorry, performance only.

thanks,

greg k-h