Re: [PATCH RT v3 1/5] rcu: Acquire RCU lock when disabling BHs

Scott Wood
2019-09-17T14:06:48Z

On Tue, 2019-09-17 at 09:44 +0200, Sebastian Andrzej Siewior wrote:
> On 2019-09-11 17:57:25 [+0100], Scott Wood wrote:
> >
> > @@ -615,10 +645,7 @@ static inline void rcu_read_unlock(void)
> > static inline void rcu_read_lock_bh(void)
> > {
> > local_bh_disable();
> > - __acquire(RCU_BH);
> > - rcu_lock_acquire(&rcu_bh_lock_map);
> > - RCU_LOCKDEP_WARN(!rcu_is_watching(),
> > - "rcu_read_lock_bh() used illegally while idle");
> > + rcu_bh_lock_acquire();
> > }
> >
> > /*
>
> I asked previously why do you need to change rcu_read_lock_bh() and you
> replied that you don't remember:
>
> https://lore.kernel.org/linux-rt-users/b948ec6cccda31925ed8dc123bd0f55423fff3d4.camel@redhat.com/
>
> Did this change?

Sorry, I missed that you were asking about rcu_read_lock_bh() as well. I
did remove the change to rcu_read_lock_bh_held().

With this patch, local_bh_disable() calls rcu_read_lock() on RT which
handles this debug stuff. Doing it twice shouldn't be explicitly harmful,
but it's redundant, and debug kernels are slow enough as is.

-Scott