Re: [PATCH v4 05/17] irq & spin_lock: Add counted interrupt disabling/enabling

[Boqun Feng <boqun@kernel.org>]

On Wed, Aug 05, 2026 at 08:26:49PM +0530, Shrikanth Hegde wrote:
> 
> 
> On 8/5/26 7:50 PM, Boqun Feng wrote:
> > On Wed, Aug 05, 2026 at 07:40:18PM +0530, Shrikanth Hegde wrote:
> > > 
> > > Hi Boqun,
> > > 
> > > > 
> > > > Something as below? Going to send it to kernel build bot and see if it
> > > > works for all configs.
> > > > 
> > > > ----------------->8
> > > > diff --git a/include/linux/interrupt_rc.h b/include/linux/interrupt_rc.h
> > > > index b9a7f05ecf42..39f30bc65548 100644
> > > > --- a/include/linux/interrupt_rc.h
> > > > +++ b/include/linux/interrupt_rc.h
> > > > @@ -12,6 +12,7 @@
> > > >     */
> > > > 
> > > >    #include <linux/irqflags.h>
> > > > +#include <linux/debug_locks.h>
> > > >    #include <linux/preempt.h>
> > > >    #include <linux/processor.h>
> > > >    #include <linux/smp.h>
> > > > @@ -63,6 +64,12 @@ static inline void local_interrupt_disable(void)
> > > > 
> > > >           new_count = hardirq_disable_enter();
> > > > 
> > > > +       /* Is hardirq disable count overflow soon? */
> > > > +       if (IS_ENABLED(CONFIG_DEBUG_PREEMPT))
> > > > +               DEBUG_LOCKS_WARN_ON((new_count & HARDIRQ_DISABLE_MASK) +
> > > > +                                   (10 << HARDIRQ_DISABLE_SHIFT) >
> > > > +                                   HARDIRQ_DISABLE_MASK);
> > > > +
> > > 
> > > This needs a return here right? Else we will see warning for 10 times
> > > and then overflow happens and we will call _local_interrupt_disable. No?

Oh, seems I overlooked something... could you elaborate on this? What's
the scenario in your mind? You said we hit 10 times warning and *then*
overflow?

> > > 
> > 
> > DEBUG_LOCKS_WARN_ON() uses debug_locks_off() to avoid this, so we won't
> > see it 10 times. The reason not using return here, because we would
> > introduce unpaired local_interrupt_disable() if we returned:
> 
> Yes, it could be a weird case if the overflow actually happens.
> So just warning maybe enough to catch such callers.
> 
> Maybe your kunit test can actually help test the behavior with the loop
> count.
> 
> > 
> > 	// hardirq disable count is n
> > 	local_interrupt_disable(); // hardirq disable count is n + 1
> > 
> > 	local_interrupt_disable(); <- trigger the warning, if we return
> > 				   // hardirq disable count is n + 1
> 
> Likely i am missing to understand.
> 

No, it was me who misunderstand ;-)

> Isn't the count incremented earlier than return?
> I.e even if return happens it should be n + 2 right?
> 

Yeah, you're right, but then why do we want to return earlier? Since the
following if:

	if ((new_count & HARDIRQ_DISABLE_MASK) == HARDIRQ_DISABLE_OFFSET)

will be false, and we will just return from the function, no?

Regards,
Boqun

> > 
> > 	local_interrupt_enable(); // hardirq disable count is n
> > 
> > 	local_interrupt_enable(); // hardirq disable count is n - 1
> > 
> > Regards,
> > Boqun
> > 
> > > Not sure, if below is any better? (Igore whitespace mangling)
> > > 
> > > if (IS_ENABLED(CONFIG_DEBUG_PREEMPT) &&
> > >      DEBUG_LOCKS_WARN_ON((preempt_count() & HARDIRQ_DISABLE_MASK) >=
> > > 			 HARDIRQ_DISABLE_MASK - (10 << HARDIRQ_DISABLE_SHIFT)))
> > > 	return;
> > > 
> > > >           /* Interrupts can happen here, but it's OK, see __irq_exit_rcu(). */
> > > > 
> > > >           if ((new_count & HARDIRQ_DISABLE_MASK) == HARDIRQ_DISABLE_OFFSET)
> > > > @@ -73,6 +80,11 @@ static inline void local_interrupt_enable(void)
> > > >    {
> > > >           int new_count;
> > > > 
> > > > +       /* Unpaired local_interrupt_enable()? Warn and abort. */
> > > > +       if (IS_ENABLED(CONFIG_DEBUG_PREEMPT) &&
> > > > +           DEBUG_LOCKS_WARN_ON((preempt_count() & HARDIRQ_DISABLE_MASK) == 0))
> > > > +               return;
> > > > +
> > > >           new_count = hardirq_disable_exit();
> > > > 
> > > >           if ((new_count & HARDIRQ_DISABLE_MASK) == 0) >
> > > 
>