Re: [PATCH tip/core/rcu 06/19] rcu: Add warning to detect half-interrupts

From: Joel Fernandes
Date: Mon Mar 11 2019 - 09:39:44 EST

On Wed, Aug 29, 2018 at 03:20:34PM -0700, Paul E. McKenney wrote:
> 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:
> 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.

Hi Paul and everyone,
I was thinking some more about this patch and whether we can simplify this code
much in 2021. Since 2021 is a bit far away, I thought working on it in again to
keep it fresh in memory is a good idea ;-)

To me it seems we cannot easily combine the counters (dynticks_nesting and
dynticks_nmi_nesting) even if we confirmed that there is no possibility of a
half-interrupt scenario (assuming simplication means counter combining like
Byungchul tried to do in The reason is because these
2 counters need to be tracked separately as they are used differently in the
following function:

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;

dynticks_nesting actually tracks if we entered/exited idle or user mode.

dynticks_nmi_nesting tracks if we entered/exited interrupts.

We have to do the "dynticks_nmi_nesting <= 1" check because
rcu_is_cpu_rrupt_from_idle() can possibly be called from an interrupt itself
(like timer) so we discount 1 interrupt, and, the "dynticks_nesting <= 0"
check is because the CPU MUST be in user or idle for the check to return
true. We can't really combine these two into one counter then I think because
they both convey different messages.

The only simplication we can do, is probably the "crowbar" updates to
dynticks_nmi_nesting can be removed from rcu_eqs_enter/exit once we confirm
no more half-interrupts are possible. Which might still be a worthwhile thing
to do (while still keeping both counters separate).

However, I think we could combine the counters and lead to simplying the code
in case we implement rcu_is_cpu_rrupt_from_idle differently such that it does
not need the counters but NOHZ_FULL may take issue with that since it needs
rcu_user_enter->rcu_eqs_enter to convey that the CPU is "RCU"-idle.

Actually, I had another question... rcu_user_enter() is a NOOP in !NOHZ_FULL config.
In this case I was wondering if the the warning Paul added (in the patch I'm replying to)
will really get fired for half-interrupts. The vast majority of the systems I believe are
This is what a half-interrupt really looks like right? Please correct me if I'm wrong:
rcu_irq_enter() [half interrupt causes an exception and thus rcu_irq_enter]
rcu_user_enter() [due to usermode upcall]
(no more rcu_irq_exit() - hence half an interrupt)

But the rcu_user_enter()/exit is a NOOP in some configs, so will the warning in
rcu_eqs_e{xit,nter} really do anything?

Or was the idea with adding the new warnings, that they would fire the next
time rcu_idle_enter/exit is called? Like for example:

rcu_irq_enter() [This is due to half-interrupt]
rcu_idle_enter() [Eventually we enter the idle loop at some point
after the half-interrupt and the rcu_eqs_enter()
would "crowbar" the dynticks_nmi_nesting counter to 0].


- Joel

> Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Reported-by: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> Reported-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> ---
> kernel/rcu/tree.c | 2 ++
> 1 file changed, 2 insertions(+)
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index dc041c2afbcc..d2b6ade692c9 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -714,6 +714,7 @@ static void rcu_eqs_enter(bool user)
> struct rcu_dynticks *rdtp;
> rdtp = this_cpu_ptr(&rcu_dynticks);
> + WARN_ON_ONCE(rdtp->dynticks_nmi_nesting != DYNTICK_IRQ_NONIDLE);
> WRITE_ONCE(rdtp->dynticks_nmi_nesting, 0);
> rdtp->dynticks_nesting == 0);
> @@ -895,6 +896,7 @@ static void rcu_eqs_exit(bool user)
> trace_rcu_dyntick(TPS("End"), rdtp->dynticks_nesting, 1, rdtp->dynticks);
> WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
> WRITE_ONCE(rdtp->dynticks_nesting, 1);
> + WARN_ON_ONCE(rdtp->dynticks_nmi_nesting);
> WRITE_ONCE(rdtp->dynticks_nmi_nesting, DYNTICK_IRQ_NONIDLE);
> }
> --
> 2.17.1