linux-kernel - [PATCH tip/core/rcu 10/21] locking: Remove smp_read_barrier_depends() from queued_spin_lock

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Message-Id: <1512157876-24665-10-git-send-email-paulmck@linux.vnet.ibm.com>
Date:   Fri,  1 Dec 2017 11:51:05 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: [PATCH tip/core/rcu 10/21] locking: Remove smp_read_barrier_depends() from queued_spin_lock_slowpath()

Queued spinlocks are not used by DEC Alpha, and furthermore operations
such as READ_ONCE() and release/relaxed RMW atomics are being changed
to imply smp_read_barrier_depends().  This commit therefore removes the
now-redundant smp_read_barrier_depends() from queued_spin_lock_slowpath(),
and adjusts the comments accordingly.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>
---
 kernel/locking/qspinlock.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 294294c71ba4..38ece035039e 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -170,7 +170,7 @@ static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
  * @tail : The new queue tail code word
  * Return: The previous queue tail code word
  *
- * xchg(lock, tail)
+ * xchg(lock, tail), which heads an address dependency
  *
  * p,*,* -> n,*,* ; prev = xchg(lock, node)
  */
@@ -409,13 +409,11 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 	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.
+		 * 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 subsequent access to @prev happens after.
 		 */
-		smp_read_barrier_depends();
 
 		WRITE_ONCE(prev->next, node);
 
-- 
2.5.2