On Sat, Jun 23, 2018 at 10:56:34AM -0700, Paul E. McKenney wrote: > On Sat, Jun 23, 2018 at 08:48:39AM -0700, Joel Fernandes wrote: > > On Fri, Jun 22, 2018 at 02:16:00PM -0700, Paul E. McKenney 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. > > > > Hi Paul, I am Ok with adding a warning for a couple of releases if you and > > others are Ok with it, how about something like this? Feel free to use the > > diff as a starting point or a different approach if you/others prefer > > something else. Thanks. > > A few years rather than a few releases, but yes. ;-) > The checks would need to go just before the "crowbar" stores.
Ok. I guess that would work too. > I will put something together after Byungchul's patches in this area have > had time to burn in for a few days. Sounds great, thanks! - Joel
