linux-kernel - Re: [PATCH tip/core/rcu 2/3] srcu: Force full grace-period ordering

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Date:   Sun, 15 Jan 2017 08:57:11 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, jiangshanlai@...il.com,
        dipankar@...ibm.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
        fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
        will.deacon@....com, boqun.feng@...ux.vnet.ibm.com,
        linuxppc-dev@...ts.ozlabs.org,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH tip/core/rcu 2/3] srcu: Force full grace-period ordering


* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> On Sun, Jan 15, 2017 at 08:11:23AM +0100, Ingo Molnar wrote:
> > 
> > * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> > 
> > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > > index 357b32aaea48..5fdfe874229e 100644
> > > --- a/include/linux/rcupdate.h
> > > +++ b/include/linux/rcupdate.h
> > > @@ -1175,11 +1175,11 @@ do { \
> > >   * if the UNLOCK and LOCK are executed by the same CPU or if the
> > >   * UNLOCK and LOCK operate on the same lock variable.
> > >   */
> > > -#ifdef CONFIG_PPC
> > > +#ifdef CONFIG_ARCH_WEAK_RELACQ
> > >  #define smp_mb__after_unlock_lock()	smp_mb()  /* Full ordering for lock. */
> > > -#else /* #ifdef CONFIG_PPC */
> > > +#else /* #ifdef CONFIG_ARCH_WEAK_RELACQ */
> > >  #define smp_mb__after_unlock_lock()	do { } while (0)
> > > -#endif /* #else #ifdef CONFIG_PPC */
> > > +#endif /* #else #ifdef CONFIG_ARCH_WEAK_RELACQ */
> > >  
> > >  
> > 
> > So at the risk of sounding totally pedantic, why not structure it like the 
> > existing smp_mb__before/after*() primitives in barrier.h?
> > 
> > That allows asm-generic/barrier.h to pick up the definition - for example in the 
> > case of smp_acquire__after_ctrl_dep() we do:
> > 
> >  #ifndef smp_acquire__after_ctrl_dep
> >  #define smp_acquire__after_ctrl_dep()           smp_rmb()
> >  #endif
> > 
> > Which allows Tile to relax it:
> > 
> >   arch/tile/include/asm/barrier.h:#define smp_acquire__after_ctrl_dep()   barrier()
> > 
> > I.e. I'd move the API definition out of rcupdate.h and into barrier.h - even 
> > though tree-RCU is the only user of this barrier type.
> 
> I wouldn't have any problem with that, however, some time back it was
> moved into RCU because (you guessed it!) RCU is the only user.  ;-)

Indeed ...

[sounds of rummaging around in the Git tree]

I found this commit of yours from ancient history (more than a year ago!):

  commit 12d560f4ea87030667438a169912380be00cea4b
  Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
  Date:   Tue Jul 14 18:35:23 2015 -0700

    rcu,locking: Privatize smp_mb__after_unlock_lock()
    
    RCU is the only thing that uses smp_mb__after_unlock_lock(), and is
    likely the only thing that ever will use it, so this commit makes this
    macro private to RCU.
    
    Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
    Cc: Will Deacon <will.deacon@....com>
    Cc: Peter Zijlstra <peterz@...radead.org>
    Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
    Cc: "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>

So I concur and I'm fine with your patch - or with the status quo code as well.

Thanks,

	Ingo