On Sat, Jun 23, 2018 at 10:53:56AM -0700, Paul E. McKenney wrote:
> On Fri, Jun 22, 2018 at 03:03:35PM -0700, Andy Lutomirski wrote:
> > On Fri, Jun 22, 2018 at 2:14 PM Paul E. McKenney
> > <paul...@linux.vnet.ibm.com> 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.
> >
> > 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>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 6c5a7f0daadc..37ae0d77854d 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);
}
