Re: [PATCH] Documentation: coding-style: don't encourage WARN*()

[Laurent Pinchart]

Hi Alex,

Thank you for the patch.

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 <elder@xxxxxxxxxx>
> ---
>  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.

>  
>  Use BUILD_BUG_ON() for compile-time assertions
>  **********************************************

-- 
Regards,

Laurent Pinchart