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

From: "Paul E. McKenney" <[email-masked]>
Date: Sat, 14 Jan 2017 13:41:59 -0800

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 <[email-masked]> 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 <[email-masked]>
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 <[email-masked]>
    Signed-off-by: Paul E. McKenney <[email-masked]>
    Cc: Peter Zijlstra <[email-masked]>
    Cc: Will Deacon <[email-masked]>
    Cc: Boqun Feng <[email-masked]>
    Cc: Benjamin Herrenschmidt <[email-masked]>
    Cc: Paul Mackerras <[email-masked]>
    Cc: Michael Ellerman <[email-masked]>
    Cc: <[email-masked]>

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 */