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

[Ingo Molnar <mingo@kernel.org>]

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

> On Sun, Jan 15, 2017 at 08:11:23AM +0100, Ingo Molnar wrote:
> > 
> > * Paul E. McKenney <paulmck@linux.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@linux.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@linux.vnet.ibm.com>
    Cc: Will Deacon <will.deacon@arm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    Cc: "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>

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

Thanks,

	Ingo