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

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

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 <elder@linaro.org>
> > > > > ---
> > > > >  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