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

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Message-ID: <63b430ca2f067e61bef1c387fad782ab4a2c1ed3.camel@redhat.com>
Date:   Tue, 17 Sep 2019 09:06:28 -0500
From:   Scott Wood <swood@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>
Subject: Re: [PATCH RT v3 1/5] rcu: Acquire RCU lock when disabling BHs

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