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

[Byungchul Park]

On Mon, Mar 11, 2019 at 09:39:39AM -0400, Joel Fernandes wrote:
> 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:
> > 
> > http://lkml.kernel.org/r/20111014170019.GE2428@xxxxxxxxxxxxxxxxxx
> > 
> > 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 https://goo.gl/X1U77X). The reason is because these
> 2 counters need to be tracked separately as they are used differently in the
> following function:

Hi Joel and Paul,

I always love the way to logically approach problems so I'm a fan of
all your works :) But I'm JUST curious about something here. Why can't
we combine them the way I tried even if we confirm no possibility of
half-interrupt? IMHO, the only thing we want to know through calling
rcu_is_cpu_rrupt_from_idle() is whether the interrupt comes from
RCU-idle or not - of course assuming the caller context always be an
well-defined interrupt context like e.g. the tick handler.

So the function can return true if the caller is within a RCU-idle
region except a well-known single interrupt nested.

Of course, now that we cannot confirm it yet, the crowbar is necessary.
But does it still have a problem even after confirming it? Why? What am
I missing? Could you explain why for me? :(

Thanks,
Byungchul

> 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
> NOHZ_IDLE not NOHZ_FULL.
> 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]
> rcu_user_exit()
> (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].
> 
> thanks!
> 
>  - 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);
> >  	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> >  		     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
> >