On Thu, Jun 28, 2018 at 01:02:05PM -0700, Paul E. McKenney wrote: > > > <paul...@linux.vnet.ibm.com> wrote: [..] > > > > > > > > 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! > > And here is a prototype patch. > > Thanx, Paul > > ------------------------------------------------------------------------ > > commit ef544593a7bcad74628fa0537badc49dce1f2d95 > Author: Paul E. McKenney <paul...@linux.vnet.ibm.com> > Date: Thu Jun 28 12:45:23 2018 -0700 > > rcu: Add warning to detect half-interrupts > > RCU's dyntick-idle code is written to tolerate half-interrupts, that it, > either an interrupt that invokes rcu_irq_enter() but never invokes the > corresponding rcu_irq_exit() on the one hand, or an interrupt that never > invokes rcu_irq_enter() but does invoke the "corresponding" rcu_irq_exit() > on the other. These things really did happen at one time, as evidenced > by this ca-2011 LKML post: > > http://lkml.kernel.org/r/20111014170019.ge2...@linux.vnet.ibm.com > > The reason why RCU tolerates half-interrupts is that usermode helpers > used exceptions to invoke a system call from within the kernel such that > the system call did a normal return (not a return from exception) to > the calling context. This caused rcu_irq_enter() to be invoked without > a matching rcu_irq_exit(). However, usermode helpers have since been > rewritten to make much more housebroken use of workqueues, kernel threads, > and do_execve(), and therefore should no longer produce half-interrupts. > No one knows of any other source of half-interrupts, but then again, > no one seems insane enough to go audit the entire kernel to verify that > half-interrupts really are a relic of the past. > > This commit therefore adds a pair of WARN_ON_ONCE() calls that will > trigger in the presence of half interrupts, which the code will continue > to handle correctly. If neither of these WARN_ON_ONCE() trigger by > mid-2021, then perhaps RCU can stop handling half-interrupts, which > would be a considerable simplification. > > Reported-by: Steven Rostedt <rost...@goodmis.org> > Reported-by: Joel Fernandes <j...@joelfernandes.org> > Reported-by: Andy Lutomirski <l...@kernel.org> > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Looks good to me! Reviewed-by: Joel Fernandes (Google) <j...@joelfernandes.org> thanks, - Joel
