On Fri, Mar 22, 2019 at 09:29:39PM -0400, Joel Fernandes (Google) wrote: > In the future we would like to combine the dynticks and dynticks_nesting > counters thus leading to simplifying the code. At the moment we cannot > do that due to concerns about usermode upcalls appearing to RCU as half > of an interrupt. Byungchul tried to do it in [1] but the > "half-interrupt" concern was raised. It is half because, what RCU > expects is rcu_irq_enter() and rcu_irq_exit() pairs when the usermode > exception happens. However, only rcu_irq_enter() is observed. This > concern may not be valid anymore, but at least it used to be the case. > > Out of abundance of caution, Paul added warnings [2] in the RCU code > which if not fired by 2021 may allow us to assume that such > half-interrupt scenario cannot happen any more, which can lead to > simplification of this code. > > Summary of the changes are the following: > > (1) In preparation for this combination of counters in the future, we > first need to first be sure that rcu_rrupt_from_idle cannot be called > from anywhere but a hard-interrupt because previously, the comments > suggested otherwise so let us be sure. We discussed this here [3]. We > use the services of lockdep to accomplish this. > > (2) Further rcu_rrupt_from_idle() is not explicit about how it is using > the counters which can lead to weird future bugs. This patch therefore > makes it more explicit about the specific counter values being tested > > (3) Lastly, we check for counter underflows just to be sure these are > not happening, because the previous code in rcu_rrupt_from_idle() was > allowing the case where the counters can underflow, and the function > would still return true. Now we are checking for specific values so let > us be confident by additional checking, that such underflows don't > happen. Any case, if they do, we should fix them and the screaming > warning is appropriate. All these checks checks are NOOPs if PROVE_RCU > and PROVE_LOCKING are disabled. > > [1] https://lore.kernel.org/patchwork/patch/952349/ > [2] Commit e11ec65cc8d6 ("rcu: Add warning to detect half-interrupts") > [3] https://lore.kernel.org/lkml/20190312150514.GB249405@xxxxxxxxxx/ > > Cc: byungchul.park@xxxxxxx > Cc: kernel-team@xxxxxxxxxxx > Cc: rcu@xxxxxxxxxxxxxxx > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > --- > kernel/rcu/tree.c | 21 +++++++++++++++++---- > 1 file changed, 17 insertions(+), 4 deletions(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 9180158756d2..d94c8ed29f6b 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -381,16 +381,29 @@ static void __maybe_unused rcu_momentary_dyntick_idle(void) > } > > /** > - * rcu_is_cpu_rrupt_from_idle - see if idle or immediately interrupted from idle > + * rcu_is_cpu_rrupt_from_idle - see if interrupted from idle > * > - * If the current CPU is idle or running at a first-level (not nested) > + * If the current CPU is idle and running at a first-level (not nested) > * interrupt from idle, return true. The caller must have at least > * disabled preemption. > */ > static int rcu_is_cpu_rrupt_from_idle(void) > { > - return __this_cpu_read(rcu_data.dynticks_nesting) <= 0 && > - __this_cpu_read(rcu_data.dynticks_nmi_nesting) <= 1; > + /* Called only from within the scheduling-clock interrupt */ > + lockdep_assert_in_irq(); > + > + /* Check for counter underflows */ > + RCU_LOCKDEP_WARN( > + (__this_cpu_read(rcu_data.dynticks_nesting) < 0) && > + (__this_cpu_read(rcu_data.dynticks_nmi_nesting) < 0), This condition for the warning is supposed to be || instead of &&. Sorry. Or, I will just use 2 RCU_LOCKDEP_WARN(s) here, that's better. thanks, - Joel
