linux-kernel - Re: [PATCH v2 tip/core/rcu 02/39] rcu: Make arch select smp_mb__after_unlock

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Date:   Mon, 17 Apr 2017 17:19:27 -0700
From:   Josh Triplett <josh@...htriplett.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, bobby.prani@...il.com,
        Will Deacon <will.deacon@....com>,
        Boqun Feng <boqun.feng@...ux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 tip/core/rcu 02/39] rcu: Make arch select
 smp_mb__after_unlock_lock() strength

On Mon, Apr 17, 2017 at 04:44:49PM -0700, Paul E. McKenney wrote:
> 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 ARCH_WEAK_RELEASE_ACQUIRE, has PPC select it,
> and bases the definition of smp_mb__after_unlock_lock() on this new
> ARCH_WEAK_RELEASE_ACQUIRE 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>
> Acked-by: Michael Ellerman <mpe@...erman.id.au>
> Cc: <linuxppc-dev@...ts.ozlabs.org>

Seems sensible.

Reviewed-by: Josh Triplett <josh@...htriplett.org>

>  arch/Kconfig             | 3 +++
>  arch/powerpc/Kconfig     | 1 +
>  include/linux/rcupdate.h | 6 +++---
>  3 files changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index cd211a14a88f..adefaf344239 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -320,6 +320,9 @@ config HAVE_CMPXCHG_LOCAL
>  config HAVE_CMPXCHG_DOUBLE
>  	bool
>  
> +config ARCH_WEAK_RELEASE_ACQUIRE
> +	bool
> +
>  config ARCH_WANT_IPC_PARSE_VERSION
>  	bool
>  
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 97a8bc8a095c..7a5c9b764cd2 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -99,6 +99,7 @@ config PPC
>  	select ARCH_USE_BUILTIN_BSWAP
>  	select ARCH_USE_CMPXCHG_LOCKREF		if PPC64
>  	select ARCH_WANT_IPC_PARSE_VERSION
> +	select ARCH_WEAK_RELEASE_ACQUIRE
>  	select BINFMT_ELF
>  	select BUILDTIME_EXTABLE_SORT
>  	select CLONE_BACKWARDS
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index de88b33c0974..e6146d0074f8 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -1127,11 +1127,11 @@ 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_RELEASE_ACQUIRE
>  #define smp_mb__after_unlock_lock()	smp_mb()  /* Full ordering for lock. */
> -#else /* #ifdef CONFIG_PPC */
> +#else /* #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
>  #define smp_mb__after_unlock_lock()	do { } while (0)
> -#endif /* #else #ifdef CONFIG_PPC */
> +#endif /* #else #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
>  
>  
>  #endif /* __LINUX_RCUPDATE_H */
> -- 
> 2.5.2
>