On Mon, Apr 15, 2024 at 11:25:29AM +0300, Laurent Pinchart wrote: > Hi Greg, > > 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@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. > > > > 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. thanks, greg k-h
