On Fri, Jun 22, 2018 at 03:03:35PM -0700, Andy Lutomirski wrote: > On Fri, Jun 22, 2018 at 2:14 PM Paul E. McKenney > <paul...@linux.vnet.ibm.com> wrote: > > > > On Fri, Jun 22, 2018 at 05:00:42PM -0400, Steven Rostedt wrote: > > > On Fri, 22 Jun 2018 13:58:13 -0700 > > > "Paul E. McKenney" <paul...@linux.vnet.ibm.com> wrote: > > > > > > > Something like this: > > > > > > > > IRQ entered > > > > > > > > And never exited. Ever. I actually saw this in 2011. > > > > > > I still believe this was actually a bug. And perhaps you made the RCU > > > code robust enough to handle this bug ;-) > > > > Welcome to my world! > > > > But I recall it being used in several places, so if it was a bug, it > > was an intentional bug. Probably the worst kind. > > > > Sort of like nested NMIs and interrupts within NMI handlers. ;-) > > > > > > Or something like this: > > > > > > > > IRQ exited > > > > > > > > Without a corresponding IRQ enter. > > > > > > > > The current code handles both of these situations, at least assuming > > > > that the interrupt entry/exit happens during a non-idle period. > > > > > > > > > > So why this function-call structure? Well, you see, NMI handlers > > > > > > can > > > > > > take what appear to RCU to be normal interrupts... > > > > > > > > > > > > (And I just added that fun fact to Requirements.html.) > > > > > > > > > > Yes, I'll definitely go through all the interrupt requirements in the > > > > > doc and > > > > > thanks for referring me to it. > > > > > > > > My concern may well be obsolete. It would be good if it was! ;-) > > > > > > I'd love to mandate that irq_enter() must be paired with irq_exit(). I > > > don't really see any rationale for it to be otherwise. If there is a > > > case, perhaps it needs to be fixed. > > > > Given that the usermode helpers now look to be common code using > > workqueues, kthreads, and calls to do_execve(), it might well be that > > the days of half-interrupts are behind us. > > > > But how to actually validate this? My offer of adding a WARN_ON_ONCE() > > and waiting a few years still stands, but perhaps you have a better > > approach. > > I think you should add a WARN_ON_ONCE(). Let's get the bugs fixed.
Or the obscure features identified, as the case may be. ;-) Either way, will do! Thanx, Paul