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

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Date:   Mon, 23 Jan 2017 19:12:03 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     paulmck@...ux.vnet.ibm.com, Ingo Molnar <mingo@...nel.org>
Cc:     boqun.feng@...ux.vnet.ibm.com, bobby.prani@...il.com,
        peterz@...radead.org, fweisbec@...il.com, dvhart@...ux.intel.com,
        jiangshanlai@...il.com, linux-kernel@...r.kernel.org,
        rostedt@...dmis.org, josh@...htriplett.org, dhowells@...hat.com,
        edumazet@...gle.com, mathieu.desnoyers@...icios.com,
        oleg@...hat.com, dipankar@...ibm.com, will.deacon@....com,
        akpm@...ux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
        tglx@...utronix.de
Subject: Re: [PATCH tip/core/rcu 2/3] srcu: Force full grace-period ordering

"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> writes:

> On Sat, Jan 14, 2017 at 11:54:17AM -0800, Paul E. McKenney wrote:
>> On Sat, Jan 14, 2017 at 10:35:50AM +0100, Ingo Molnar wrote:
>> > * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
>
> [ . . . ]
>
>> > > + */
>> > > +#ifdef CONFIG_PPC
>> > > +#define smp_mb__after_unlock_lock()	smp_mb()  /* Full ordering for lock. */
>> > > +#else /* #ifdef CONFIG_PPC */
>> > > +#define smp_mb__after_unlock_lock()	do { } while (0)
>> > > +#endif /* #else #ifdef CONFIG_PPC */
>> > 
>> > Yeah, so I realize that this was pre-existing code, but putting CONFIG_$ARCH
>> > #ifdefs into generic headers is generally frowned upon.
>> > 
>> > The canonical approach would be either to define a helper Kconfig variable that 
>> > can be set by PPC (but other architectures don't need to set it), or to expose a 
>> > suitable macro (function) for architectures to define in their barrier.h arch 
>> > header file.
>> 
>> Very well, I will add a separate commit for this.  4.11 OK?
>
> Does the patch below seem reasonable?
>
> 							Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 271c0601237c41a279f975563e13837bace0df03
> Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Date:   Sat Jan 14 13:32:50 2017 -0800
>
>     rcu: Make arch select smp_mb__after_unlock_lock() strength
>     
>     The definition of smp_mb__after_unlock_lock() is currently smp_mb()
>     for CONFIG_PPC and a no-op otherwise.  It would be better to instead
>     provide an architecture-selectable Kconfig option, and select the
>     strength of smp_mb__after_unlock_lock() based on that option.  This
>     commit therefore creates CONFIG_ARCH_WEAK_RELACQ, has PPC select it,
>     and bases the definition of smp_mb__after_unlock_lock() on this new
>     CONFIG_ARCH_WEAK_RELACQ Kconfig option.
>     
>     Reported-by: Ingo Molnar <mingo@...nel.org>
>     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>     Cc: Peter Zijlstra <peterz@...radead.org>
>     Cc: Will Deacon <will.deacon@....com>
>     Cc: Boqun Feng <boqun.feng@...ux.vnet.ibm.com>
>     Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
>     Cc: Paul Mackerras <paulus@...ba.org>
>     Cc: Michael Ellerman <mpe@...erman.id.au>
>     Cc: <linuxppc-dev@...ts.ozlabs.org>

Personally I'd call it ARCH_WEAK_RELEASE_ACQUIRE, which is longer but
clearer I think. But it's not a big deal, so which ever you prefer.

Acked-by: Michael Ellerman <mpe@...erman.id.au>

cheers