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

On Sun, Jan 15, 2017 at 08:57:11AM +0100, Ingo Molnar wrote:

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

quoted

On Sun, Jan 15, 2017 at 08:11:23AM +0100, Ingo Molnar wrote:

quoted

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

quoted

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 */
 
 

So at the risk of sounding totally pedantic, why not structure it like the 
existing smp_mb__before/after*() primitives in barrier.h?

That allows asm-generic/barrier.h to pick up the definition - for example in the 
case of smp_acquire__after_ctrl_dep() we do:

 #ifndef smp_acquire__after_ctrl_dep
 #define smp_acquire__after_ctrl_dep()           smp_rmb()
 #endif

Which allows Tile to relax it:

  arch/tile/include/asm/barrier.h:#define smp_acquire__after_ctrl_dep()   barrier()

I.e. I'd move the API definition out of rcupdate.h and into barrier.h - even 
though tree-RCU is the only user of this barrier type.

I wouldn't have any problem with that, however, some time back it was
moved into RCU because (you guessed it!) RCU is the only user.  ;-)

Indeed ...

[sounds of rummaging around in the Git tree]

I found this commit of yours from ancient history (more than a year ago!):

  commit 12d560f4ea87030667438a169912380be00cea4b
  Author: Paul E. McKenney [off-list ref]
  Date:   Tue Jul 14 18:35:23 2015 -0700

    rcu,locking: Privatize smp_mb__after_unlock_lock()
    
    RCU is the only thing that uses smp_mb__after_unlock_lock(), and is
    likely the only thing that ever will use it, so this commit makes this
    macro private to RCU.
    
    Signed-off-by: Paul E. McKenney [off-list ref]
    Cc: Will Deacon [off-list ref]
    Cc: Peter Zijlstra [off-list ref]
    Cc: Benjamin Herrenschmidt [off-list ref]
    Cc: "linux-arch@vger.kernel.org" [off-list ref]

So I concur and I'm fine with your patch - or with the status quo code as well.

I already have the patch queued, so how about I keep it if I get an ack
from the powerpc guys and drop it otherwise?

							Thanx, Paul