From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from perceval.ideasonboard.com (perceval.ideasonboard.com [213.167.242.64]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id F0A893BBCA; Mon, 15 Apr 2024 08:42:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=213.167.242.64 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1713170556; cv=none; b=Z47Ifpjn4O42C8dicDvgdey1ykQ58I7OBrxVY94S73+r8xAIzrS34Tv27ectGqDif/qawIKUIc/nDLF3GVMX6GNryc2OIuJxfzfNUEBw2zMHzgwRCWamzZrhYmztOq8z5Mo4Zg+Z6o/uvJEHlnvMWU7kolaHOPRQhgfFoemKkvE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1713170556; c=relaxed/simple; bh=R4RhSxKXlxRSRYlUtXmi2vUBrMvRbiLLEC4+h0KivfQ=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=j1JVJshq4GvMeSDCD9gd4QGrGZl+71Mo/JsuDOz7yXtS3GUm39FH6Ok+Ao5D8+lTy8H8VoQDpxQJ30Zig6rh9eVorjQpBq0V50tuZhWsfNwGOfL1tslaYoaC/cf/pXfHGCXNjiZ4LErA59NUGc2m9pMh7uO2fB1iV+c/TY7kTWg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=ideasonboard.com; spf=pass smtp.mailfrom=ideasonboard.com; dkim=pass (1024-bit key) header.d=ideasonboard.com header.i=@ideasonboard.com header.b=YwurJmVr; arc=none smtp.client-ip=213.167.242.64 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=ideasonboard.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=ideasonboard.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=ideasonboard.com header.i=@ideasonboard.com header.b="YwurJmVr" Received: from pendragon.ideasonboard.com (117.145-247-81.adsl-dyn.isp.belgacom.be [81.247.145.117]) by perceval.ideasonboard.com (Postfix) with ESMTPSA id B163F5B2; Mon, 15 Apr 2024 10:41:46 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ideasonboard.com; s=mail; t=1713170506; bh=R4RhSxKXlxRSRYlUtXmi2vUBrMvRbiLLEC4+h0KivfQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=YwurJmVrspapPm+kBHwP0RV3Q4ljefqSl+vnBSR0Yjn/liPfGDnARXb2YY0HLYAtl ROYFjB058LVmeTPkSM4s4IgfWgwfu5D/rxwVB742Lzc/jdcbO4WBeNGkAruaI9wvzC eiPZSWwr6eYEMjYEok1ouROhAD995bTKhLZrsqVU= Date: Mon, 15 Apr 2024 11:42:24 +0300 From: Laurent Pinchart To: Greg KH Cc: Alex Elder , corbet@lwn.net, workflows@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] Documentation: coding-style: don't encourage WARN*() Message-ID: <20240415084224.GE25078@pendragon.ideasonboard.com> References: <20240414170850.148122-1-elder@linaro.org> <20240414194835.GA12561@pendragon.ideasonboard.com> <2024041503-affidavit-stopwatch-72d7@gregkh> <20240415082529.GD25078@pendragon.ideasonboard.com> <2024041511-goldmine-persevere-68f4@gregkh> Precedence: bulk X-Mailing-List: workflows@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <2024041511-goldmine-persevere-68f4@gregkh> On Mon, Apr 15, 2024 at 10:33:42AM +0200, Greg KH wrote: > On Mon, Apr 15, 2024 at 11:25:29AM +0300, Laurent Pinchart wrote: > > On Mon, Apr 15, 2024 at 07:21:37AM +0200, Greg KH wrote: > > > On Sun, Apr 14, 2024 at 10:48:35PM +0300, Laurent Pinchart wrote: > > > > On Sun, Apr 14, 2024 at 12:08:50PM -0500, Alex Elder wrote: > > > > > Several times recently Greg KH has admonished that variants of WARN() > > > > > should not be used, because when the panic_on_warn kernel option is set, > > > > > their use can lead to a panic. His reasoning was that the majority of > > > > > Linux instances (including Android and cloud systems) run with this option > > > > > enabled. And therefore a condition leading to a warning will frequently > > > > > cause an undesirable panic. > > > > > > > > > > The "coding-style.rst" document says not to worry about this kernel > > > > > option. Update it to provide a more nuanced explanation. > > > > > > > > > > Signed-off-by: Alex Elder > > > > > --- > > > > > Documentation/process/coding-style.rst | 21 +++++++++++---------- > > > > > 1 file changed, 11 insertions(+), 10 deletions(-) > > > > > > > > > > diff --git a/Documentation/process/coding-style.rst b/Documentation/process/coding-style.rst > > > > > index 9c7cf73473943..bce43b01721cb 100644 > > > > > --- a/Documentation/process/coding-style.rst > > > > > +++ b/Documentation/process/coding-style.rst > > > > > @@ -1235,17 +1235,18 @@ example. Again: WARN*() must not be used for a condition that is expected > > > > > to trigger easily, for example, by user space actions. pr_warn_once() is a > > > > > possible alternative, if you need to notify the user of a problem. > > > > > > > > > > -Do not worry about panic_on_warn users > > > > > -************************************** > > > > > +The panic_on_warn kernel option > > > > > +******************************** > > > > > > > > > > -A few more words about panic_on_warn: Remember that ``panic_on_warn`` is an > > > > > -available kernel option, and that many users set this option. This is why > > > > > -there is a "Do not WARN lightly" writeup, above. However, the existence of > > > > > -panic_on_warn users is not a valid reason to avoid the judicious use > > > > > -WARN*(). That is because, whoever enables panic_on_warn has explicitly > > > > > -asked the kernel to crash if a WARN*() fires, and such users must be > > > > > -prepared to deal with the consequences of a system that is somewhat more > > > > > -likely to crash. > > > > > +Note that ``panic_on_warn`` is an available kernel option. If it is enabled, > > > > > +a WARN*() call whose condition holds leads to a kernel panic. Many users > > > > > +(including Android and many cloud providers) set this option, and this is > > > > > +why there is a "Do not WARN lightly" writeup, above. > > > > > + > > > > > +The existence of this option is not a valid reason to avoid the judicious > > > > > +use of warnings. There are other options: ``dev_warn*()`` and ``pr_warn*()`` > > > > > +issue warnings but do **not** cause the kernel to crash. Use these if you > > > > > +want to prevent such panics. > > > > > > > > Those options are not equivalent, they print a single message, which is > > > > much easier to ignore. WARN() is similar to -Werror in some sense, it > > > > pushes vendors to fix the warnings. I have used WARN() in the past to > > > > indicate usage of long-deprecated APIs that we were getting close to > > > > removing for instance. dev_warn() wouldn't have had the same effect. > > > > > > If you want to reboot a box because someone called an "improper" api, > > > > I don't "want" to reboot. It came as a side effect when panic_on_warn > > was added, and worsened when its adoption increased. I won't argued for > > or against panic_on_warn, but WARN() serves some use cases today that I > > consider valid. If we want to discourage its usage, we need another API > > to cover those use cases. > > > > > then sure, use WARN(), but that feels like a really bad idea. Just > > > remove the api and fix up all in-kernel users instead. Why wait? > > > > There are multiple use cases. One of them is to make sure no new user of > > the old, deprecated behaviour is introduced. This is especially > > important when driver development spans multiple kernel releases, the > > development can start before the API behaviour changes, with the driver > > merged after the API change. This is something we've done multiple times > > in V4L2. > > > > > If you want to show a traceback, then just print that out, but I've seen > > > that totally ignored as well, removing the api is usually the only way > > > to get people to actually notice, as then their builds break. > > > > Does your experience tell that tracebacks are routinely ignored during > > development too, not just in production ? > > Yes, we have done this in the past in some driver core apis and nothing > ever changed until we actually deleted the apis. Let's keep WARN() + panic_on_warn then, it should help making people notice :-) Jokes aside, if we want to discourage new users of WARN() because of panic_on_warn, I'd like a WARN_NO_PANIC(). -- Regards, Laurent Pinchart