On Wed, 2021-07-07 at 17:46 +0200, Greg KH wrote: > On Wed, Jul 07, 2021 at 11:34:17AM -0400, J. Bruce Fields wrote: > > On Wed, Jul 07, 2021 at 05:31:06PM +0200, Greg KH wrote: > > > On Wed, Jul 07, 2021 at 11:19:36AM -0400, J. Bruce Fields wrote: > > > > On Wed, Jul 07, 2021 at 05:06:45PM +0200, Greg KH wrote: > > > > > On Wed, Jul 07, 2021 at 09:51:29AM -0400, J. Bruce Fields wrote: > > > > > > On Wed, Jul 07, 2021 at 07:40:47AM -0400, Jeff Layton wrote: > > > > > > > On Wed, 2021-07-07 at 12:51 +0200, Greg KH wrote: > > > > > > > > On Wed, Jul 07, 2021 at 06:44:42AM -0400, Jeff Layton wrote: > > > > > > > > > On Wed, 2021-07-07 at 08:05 +0200, Greg KH wrote: > > > > > > > > > > On Wed, Jul 07, 2021 at 10:35:47AM +0800, Desmond Cheong Zhi Xi wrote: > > > > > > > > > > > + WARN_ON_ONCE(irqs_disabled()); > > > > > > > > > > > > > > > > > > > > If this triggers, you just rebooted the box :( > > > > > > > > > > > > > > > > > > > > Please never do this, either properly handle the problem and return an > > > > > > > > > > error, or do not check for this. It is not any type of "fix" at all, > > > > > > > > > > and at most, a debugging aid while you work on the root problem. > > > > > > > > > > > > > > > > > > > > thanks, > > > > > > > > > > > > > > > > > > > > greg k-h > > > > > > > > > > > > > > > > > > Wait, what? Why would testing for irqs being disabled and throwing a > > > > > > > > > WARN_ON in that case crash the box? > > > > > > > > > > > > > > > > If panic-on-warn is enabled, which is a common setting for systems these > > > > > > > > days. > > > > > > > > > > > > > > Ok, that makes some sense. > > > > > > > > > > > > Wait, I don't get it. > > > > > > > > > > > > How are we supposed to decide when to use WARN, when to use BUG, and > > > > > > when to panic? Do we really want to treat them all as equivalent? And > > > > > > who exactly is turning on panic-on-warn? > > > > > > > > > > You never use WARN or BUG, unless the system is so messed up that you > > > > > can not possibly recover from the issue. > > > > > > > > I've heard similar advice for BUG before, but this is the first I've > > > > heard it for WARN. Do we have any guidelines for how to choose between > > > > WARN and BUG? > > > > > > Never use either :) > > > > I can't tell if you're kidding. > > I am not. > > > Is there some plan to remove them? > > Over time, yes. And any WARN that userspace can ever hit should be > removed today. > > > There are definitely cases where I've been able to resolve a problem > > more quickly because I got a backtrace from a WARN. > > If you want a backtrace, ask for that, recover from the error, and move > on. Do not allow userspace to reboot a machine for no good reason as > again, panic-on-warn is a common setting that people use now. > > This is what all of the syzbot work has been doing, it triggers things > that cause WARN() to be hit and so we have to fix them. > This seems really draconian. Clearly we do want to fix things that show a WARN (otherwise we wouldn't bother warning about it), but I don't think that's a reason to completely avoid them. My understanding has always been: BUG: for when you reach some condition where the kernel (probably) can't carry on WARN: for when you reach some condition that is problematic but where the machine can probably soldier on. Over the last several years, I've changed a lot of BUGs into WARNs to avoid crashing the box unnecessarily. If someone is setting panic_on_warn, then aren't they just getting what they asked for? While I don't feel that strongly about this particular WARN in this patch, it seems like a reasonable thing to do. If someone calls these functions with IRQs disabled, then they might end up with some subtle problems that could be hard to detect otherwise. -- Jeff Layton <jlayton@xxxxxxxxxx>
