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

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Message-Id: <20181108173525.GO4170@linux.ibm.com>
Date:   Thu, 8 Nov 2018 09:35:25 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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 Thu, Nov 08, 2018 at 06:15:16PM +0100, Sebastian Andrzej Siewior wrote:
> 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

I agree that tglx's patch is needed for 4.19 and earlier.  Just not for
4.20 and later.

Or am I still missing your point?

							Thanx, Paul