linux-kernel - Re: rcu: Merge RCU-bh into RCU-preempt

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Message-ID: <20181108171516.axwvjtgay4zolc56@linutronix.de>
Date:   Thu, 8 Nov 2018 18:15:16 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     tglx@...utronix.de, linux-rt-users@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: rcu: Merge RCU-bh into RCU-preempt

On 2018-11-08 08:42:47 [-0800], Paul E. McKenney wrote:
> On Thu, Nov 08, 2018 at 05:02:57PM +0100, Sebastian Andrzej Siewior wrote:
> > On 2018-11-01 16:18:04 [-0700], Paul E. McKenney wrote:
> > > The need for this goes away as of the current merge window because
> > > RCU-bh has gone away.  (Aside from still being able to do things
> > > like rcu_read_lock_bh() as a documentation device.)
> > 
> > So in -RT rcu_read_lock_bh() does
> >  { local_bh_disable() ;  rcu_read_lock() }
> > 
> > So you are saying that this is also the case in v4.20?
> 
> No, rcu_read_lock_bh() and rcu_read_unlock_bh() are unchanged in v4.20.
> With the new RCU grace-period mechanism, local_bh_disable() blocks future
> grace periods on its own.
> 
> Unless I am missing something (quite probable, actually), the v4.20
> definitions of rcu_read_lock_bh() and rcu_read_unlock_bh() should work
> as-is for -rt.

I *think* tglx made this patch, then you somehow reverted it partly [0]
and the final piece we need for RT is this gem:

  https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/tree/patches/rcu-Eliminate-softirq-processing-from-rcutree.patch?h=linux-4.19.y-rt-patches

[0] rcu: Make ksoftirqd do RCU quiescent states
    https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/tree/patches/patch-to-introduce-rcu-bh-qs-where-safe-from-softirq.patch?h=linux-4.19.y-rt-patches

> 							Thanx, Paul

Sebastian