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:

quoted

* Paul E. McKenney [off-list ref] wrote:

[ . . . ]

quoted

quoted

+ */
+#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 [off-list ref]
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 [off-list ref]
    Signed-off-by: Paul E. McKenney [off-list ref]
    Cc: Peter Zijlstra [off-list ref]
    Cc: Will Deacon [off-list ref]
    Cc: Boqun Feng [off-list ref]
    Cc: Benjamin Herrenschmidt [off-list ref]
    Cc: Paul Mackerras [off-list ref]
    Cc: Michael Ellerman [off-list ref]
    Cc: [off-list ref]

diff --git a/arch/Kconfig b/arch/Kconfig
index 99839c23d453..94dd90d33f95 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig

@@ -316,6 +316,9 @@ config HAVE_CMPXCHG_LOCAL
 config HAVE_CMPXCHG_DOUBLE
 	bool
 
+config ARCH_WEAK_RELACQ
+	bool
+
 config ARCH_WANT_IPC_PARSE_VERSION
 	bool
 

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index a8ee573fe610..e7083d27271e 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig

@@ -165,6 +165,7 @@ config PPC
 	select HAVE_ARCH_HARDENED_USERCOPY
 	select HAVE_KERNEL_GZIP
 	select HAVE_CC_STACKPROTECTOR
+	select ARCH_WEAK_RELACQ
 
 config GENERIC_CSUM
 	def_bool CPU_LITTLE_ENDIAN

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 */
 
 
 #endif /* __LINUX_RCUPDATE_H */