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

[Joel Fernandes <joel@joelfernandes.org>]

On Tue, Mar 12, 2019 at 08:20:34AM -0700, Paul E. McKenney wrote:
[snip]
> 
> >            Could we be more explicit in the code that this function can only
> > be called from an interrupt, and also we change the code comment to be more
> > clear about it (like the following diff)?
> 
> That would be good!
> 
> Nice trick on using dyntick state to check for interrupt nesting, but
> wouldn't consolidating the counters break that?  But is there a lockdep
> check for being in a hardware interrupt handler?  If not, could one
> be added?  This would have the benefit of not adding overhead to the
> scheduling-clock interrupt in production builds of the Linux kernel,
> while still finding this bug in testing.
> 
> (Another approach would be to use IS_ENABLED(CONFIG_PROVE_RCU), but
> a lockdep check would be cleaner.)

AFAICS, lockdep does not specifically track when we enter an interrupt, but
rather only tracks when interrupts are enabled/disabled.

But we could use in_irq() to find if we are in an interrupt (which uses the
preempt_count to track in HARDIRQ_MASK section of the counter).

I will add an in_irq() check that does the check only when PROVE_RCU is
enabled, and send a patch.

Thanks and it is my pleasure to look into this, quite interesting!

 - Joel