Re: [PATCH v2 tip/core/rcu 02/39] rcu: Make arch select smp_mb__after_unlock_lock() strength

[Josh Triplett]

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 
> Signed-off-by: Paul E. McKenney 
> Cc: Peter Zijlstra 
> Cc: Will Deacon 
> Cc: Boqun Feng 
> Cc: Benjamin Herrenschmidt 
> Cc: Paul Mackerras 
> Acked-by: Michael Ellerman 
> Cc: 

Seems sensible.

Reviewed-by: Josh Triplett 

>  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
> 

[PATCH v2 tip/core/rcu 02/39] rcu: Make arch select smp_mb__after_unlock_lock() strength

[Paul E. McKenney]

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 
Signed-off-by: Paul E. McKenney 
Cc: Peter Zijlstra 
Cc: Will Deacon 
Cc: Boqun Feng 
Cc: Benjamin Herrenschmidt 
Cc: Paul Mackerras 
Acked-by: Michael Ellerman 
Cc: 
---
 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