linux-kernel - [tip:locking/core] locking/pvqspinlock: Kick the PV CPU unconditionally when _Q_SLOW

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Date:	Fri, 18 Sep 2015 01:50:55 -0700
From:	tip-bot for Waiman Long <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	scott.norton@...com, torvalds@...ux-foundation.org,
	paulmck@...ux.vnet.ibm.com, dave@...olabs.net,
	peterz@...radead.org, doug.hatch@...com, Waiman.Long@....com,
	tglx@...utronix.de, mingo@...nel.org, hpa@...or.com
Subject: [tip:locking/core] locking/pvqspinlock:
  Kick the PV CPU unconditionally when _Q_SLOW_VAL

Commit-ID:  93edc8bd7750ff3cae088bfca453ea73dc9004a4
Gitweb:     http://git.kernel.org/tip/93edc8bd7750ff3cae088bfca453ea73dc9004a4
Author:     Waiman Long <Waiman.Long@....com>
AuthorDate: Fri, 11 Sep 2015 14:37:34 -0400
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 18 Sep 2015 09:27:29 +0200

locking/pvqspinlock: Kick the PV CPU unconditionally when _Q_SLOW_VAL

If _Q_SLOW_VAL has been set, the vCPU state must have been vcpu_hashed.
The extra check at the end of __pv_queued_spin_unlock() is unnecessary
and can be removed.

Signed-off-by: Waiman Long <Waiman.Long@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Davidlohr Bueso <dave@...olabs.net>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Douglas Hatch <doug.hatch@...com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Scott J Norton <scott.norton@...com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/1441996658-62854-3-git-send-email-Waiman.Long@hpe.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/locking/qspinlock_paravirt.h | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
index c8e6e9a..f0450ff 100644
--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -267,7 +267,6 @@ static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node)
 		}
 
 		if (!lp) { /* ONCE */
-			WRITE_ONCE(pn->state, vcpu_hashed);
 			lp = pv_hash(lock, pn);
 
 			/*
@@ -275,11 +274,9 @@ static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node)
 			 * when we observe _Q_SLOW_VAL in __pv_queued_spin_unlock()
 			 * we'll be sure to be able to observe our hash entry.
 			 *
-			 *   [S] pn->state
 			 *   [S] <hash>                 [Rmw] l->locked == _Q_SLOW_VAL
 			 *       MB                           RMB
 			 * [RmW] l->locked = _Q_SLOW_VAL  [L] <unhash>
-			 *                                [L] pn->state
 			 *
 			 * Matches the smp_rmb() in __pv_queued_spin_unlock().
 			 */
@@ -364,8 +361,7 @@ __visible void __pv_queued_spin_unlock(struct qspinlock *lock)
 	 * vCPU is harmless other than the additional latency in completing
 	 * the unlock.
 	 */
-	if (READ_ONCE(node->state) == vcpu_hashed)
-		pv_kick(node->cpu);
+	pv_kick(node->cpu);
 }
 /*
  * Include the architecture specific callee-save thunk of the
--
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