locking/qspinlock: Clarify xchg_tail() ordering

While going over the code I noticed that xchg_tail() is a RELEASE but
had no obvious pairing commented.

It pairs with a somewhat unique address dependency through
decode_tail().

So the store-release of xchg_tail() is paired by the address
dependency of the load of xchg_tail followed by the dereference from
the pointer computed from that load.

The @old -> @prev transformation itself is pure, and therefore does
not depend on external state, so that is immaterial wrt. ordering.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman Long <waiman.long@hpe.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/qspinlock.c

@@ -90,7 +90,7 @@ static DEFINE_PER_CPU_ALIGNED(struct mcs_spinlock, mcs_nodes[MAX_NODES]);
  * therefore increment the cpu number by one.
  */
 
-static inline u32 encode_tail(int cpu, int idx)
+static inline __pure u32 encode_tail(int cpu, int idx)
 {
 	u32 tail;
 
@@ -103,7 +103,7 @@ static inline u32 encode_tail(int cpu, int idx)
 	return tail;
 }
 
-static inline struct mcs_spinlock *decode_tail(u32 tail)
+static inline __pure struct mcs_spinlock *decode_tail(u32 tail)
 {
 	int cpu = (tail >> _Q_TAIL_CPU_OFFSET) - 1;
 	int idx = (tail &  _Q_TAIL_IDX_MASK) >> _Q_TAIL_IDX_OFFSET;
@@ -455,6 +455,8 @@ queue:
 	 * pending stuff.
 	 *
 	 * p,*,* -> n,*,*
+	 *
+	 * RELEASE, such that the stores to @node must be complete.
 	 */
 	old = xchg_tail(lock, tail);
 	next = NULL;
@@ -465,6 +467,15 @@ queue:
 	 */
 	if (old & _Q_TAIL_MASK) {
 		prev = decode_tail(old);
+		/*
+		 * The above xchg_tail() is also a load of @lock which generates,
+		 * through decode_tail(), a pointer.
+		 *
+		 * The address dependency matches the RELEASE of xchg_tail()
+		 * such that the access to @prev must happen after.
+		 */
+		smp_read_barrier_depends();
+
 		WRITE_ONCE(prev->next, node);
 
 		pv_wait_node(node, prev);