Date:	Tue, 14 Jul 2015 18:38:20 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Will Deacon <will.deacon@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [RFC PATCH v2] memory-barriers: remove
 smp_mb__after_unlock_lock()

On Tue, Jul 14, 2015 at 12:31:44PM -0700, Paul E. McKenney wrote:
> On Tue, Jul 14, 2015 at 03:12:16PM +0100, Will Deacon wrote:
> > On Tue, Jul 14, 2015 at 03:00:14PM +0100, Paul E. McKenney wrote:
> > > On Tue, Jul 14, 2015 at 01:51:46PM +0100, Will Deacon wrote:
> > > > On Tue, Jul 14, 2015 at 01:45:40PM +0100, Paul E. McKenney wrote:
> > > > > On Tue, Jul 14, 2015 at 11:04:29AM +0100, Will Deacon wrote:
> > > > > > Given that RCU is currently the only user of this barrier, how would you
> > > > > > feel about making the barrier local to RCU and not part of the general
> > > > > > memory-barrier API?
> > > > > 
> > > > > In theory, no objection.  Your thought is to leave the definitions where
> > > > > they are, mark them as being used only by RCU, and removing mention from
> > > > > memory-barriers.txt?  Or did you have something else in mind?
> > > > 
> > > > Actually, I was thinking of defining them in an RCU header file with an
> > > > #ifdef CONFIG_POWERPC for the smb_mb() version. Then you could have a big
> > > > comment describing the semantics, or put that in an RCU Documentation file
> > > > instead of memory-barriers.txt.
> > > > 
> > > > That *should* then mean we notice anybody else trying to use the barrier,
> > > > because they'd need to send patches to either add something equivalent
> > > > or move the definition out again.
> > > 
> > > My concern with this approach is that someone putting together a new
> > > architecture might miss this.  That said, this approach certainly would
> > > work for the current architectures.
> > 
> > I don't think they're any more likely to miss it than with the current
> > situation where the generic code defines the macro as a NOP unless you
> > explicitly override it.
> 
> Fair enough...

Like this?

							Thanx, Paul

------------------------------------------------------------------------

commit 695c05d4b9666c50b40a1c022678b5f6e2e3e771
Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
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 <paulmck@...ux.vnet.ibm.com>
    Cc: Will Deacon <will.deacon@....com>
    Cc: Peter Zijlstra <peterz@...radead.org>
    Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
    Cc: "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 318523872db5..eafa6a53f72c 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1854,16 +1854,10 @@ RELEASE are to the same lock variable, but only from the perspective of
 another CPU not holding that lock.  In short, a ACQUIRE followed by an
 RELEASE may -not- be assumed to be a full memory barrier.
 
-Similarly, the reverse case of a RELEASE followed by an ACQUIRE does not
-imply a full memory barrier.  If it is necessary for a RELEASE-ACQUIRE
-pair to produce a full barrier, the ACQUIRE can be followed by an
-smp_mb__after_unlock_lock() invocation.  This will produce a full barrier
-(including transitivity) if either (a) the RELEASE and the ACQUIRE are
-executed by the same CPU or task, or (b) the RELEASE and ACQUIRE act on
-the same variable.  The smp_mb__after_unlock_lock() primitive is free
-on many architectures.  Without smp_mb__after_unlock_lock(), the CPU's
-execution of the critical sections corresponding to the RELEASE and the
-ACQUIRE can cross, so that:
+Similarly, the reverse case of a RELEASE followed by an ACQUIRE does
+not imply a full memory barrier.  Therefore, the CPU's execution of the
+critical sections corresponding to the RELEASE and the ACQUIRE can cross,
+so that:
 
 	*A = a;
 	RELEASE M
@@ -1901,29 +1895,6 @@ the RELEASE would simply complete, thereby avoiding the deadlock.
 	a sleep-unlock race, but the locking primitive needs to resolve
 	such races properly in any case.
 
-With smp_mb__after_unlock_lock(), the two critical sections cannot overlap.
-For example, with the following code, the store to *A will always be
-seen by other CPUs before the store to *B:
-
-	*A = a;
-	RELEASE M
-	ACQUIRE N
-	smp_mb__after_unlock_lock();
-	*B = b;
-
-The operations will always occur in one of the following orders:
-
-	STORE *A, RELEASE, ACQUIRE, smp_mb__after_unlock_lock(), STORE *B
-	STORE *A, ACQUIRE, RELEASE, smp_mb__after_unlock_lock(), STORE *B
-	ACQUIRE, STORE *A, RELEASE, smp_mb__after_unlock_lock(), STORE *B
-
-If the RELEASE and ACQUIRE were instead both operating on the same lock
-variable, only the first of these alternatives can occur.  In addition,
-the more strongly ordered systems may rule out some of the above orders.
-But in any case, as noted earlier, the smp_mb__after_unlock_lock()
-ensures that the store to *A will always be seen as happening before
-the store to *B.
-
 Locks and semaphores may not provide any guarantee of ordering on UP compiled
 systems, and so cannot be counted on in such a situation to actually achieve
 anything at all - especially with respect to I/O accesses - unless combined
@@ -2154,40 +2125,6 @@ But it won't see any of:
 	*E, *F or *G following RELEASE Q
 
 
-However, if the following occurs:
-
-	CPU 1				CPU 2
-	===============================	===============================
-	WRITE_ONCE(*A, a);
-	ACQUIRE M		     [1]
-	WRITE_ONCE(*B, b);
-	WRITE_ONCE(*C, c);
-	RELEASE M	     [1]
-	WRITE_ONCE(*D, d);		WRITE_ONCE(*E, e);
-					ACQUIRE M		     [2]
-					smp_mb__after_unlock_lock();
-					WRITE_ONCE(*F, f);
-					WRITE_ONCE(*G, g);
-					RELEASE M	     [2]
-					WRITE_ONCE(*H, h);
-
-CPU 3 might see:
-
-	*E, ACQUIRE M [1], *C, *B, *A, RELEASE M [1],
-		ACQUIRE M [2], *H, *F, *G, RELEASE M [2], *D
-
-But assuming CPU 1 gets the lock first, CPU 3 won't see any of:
-
-	*B, *C, *D, *F, *G or *H preceding ACQUIRE M [1]
-	*A, *B or *C following RELEASE M [1]
-	*F, *G or *H preceding ACQUIRE M [2]
-	*A, *B, *C, *E, *F or *G following RELEASE M [2]
-
-Note that the smp_mb__after_unlock_lock() is critically important
-here: Without it CPU 3 might see some of the above orderings.
-Without smp_mb__after_unlock_lock(), the accesses are not guaranteed
-to be seen in order unless CPU 3 holds lock M.
-
 
 ACQUIRES VS I/O ACCESSES
 ------------------------
diff --git a/arch/powerpc/include/asm/spinlock.h b/arch/powerpc/include/asm/spinlock.h
index 4dbe072eecbe..523673d7583c 100644
--- a/arch/powerpc/include/asm/spinlock.h
+++ b/arch/powerpc/include/asm/spinlock.h
@@ -28,8 +28,6 @@
 #include <asm/synch.h>
 #include <asm/ppc-opcode.h>
 
-#define smp_mb__after_unlock_lock()	smp_mb()  /* Full ordering for lock. */
-
 #ifdef CONFIG_PPC64
 /* use 0x800000yy when locked, where yy == CPU number */
 #ifdef __BIG_ENDIAN__
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 80d974df0ea0..a9fea7395ba2 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -645,3 +645,15 @@ static inline void rcu_nocb_q_lengths(struct rcu_data *rdp, long *ql, long *qll)
 #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
 }
 #endif /* #ifdef CONFIG_RCU_TRACE */
+
+/*
+ * Place this after a lock-acquisition primitive to guarantee that
+ * an UNLOCK+LOCK pair act as a full barrier.  This guarantee applies
+ * 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
+#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 */

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