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. ---8<----------------------- diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 8788ddbc0d13..176de74f5027 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -767,6 +767,21 @@ void rcu_idle_enter(void) */ void rcu_user_enter(void) { + struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks); + + /* + * Add warning to ensure: no known instances of entering userspace from + * IRQ/NMI handlers exist today. Currently special crowbarring of the + * dynticks_nmi_nesting and maintaining of this separate counter when + * dynticks_nesting exists, is done in order to handle entering of + * userspace from an IRQ context and never returning. Lets track it for + * a couple of kernel releases and then if the warning doesn't occur, + * we can try to simplify the code and combine/eliminate the counters. + * See: http://lkml.kernel.org/r/20180622181422.gt3...@linux.vnet.ibm.com + */ + WARN_ON_ONCE(rdtp->dynticks_nmi_nesting && + rdtp->dynticks_nmi_nesting != DYNTICK_IRQ_NONIDLE); + lockdep_assert_irqs_disabled(); rcu_eqs_enter(true); }