linux-kernel - [PATCH v6 2/6] locking/pvqspinlock: Unconditional PV kick with _Q_SLOW

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Date:	Fri, 11 Sep 2015 14:37:34 -0400
From:	Waiman Long <Waiman.Long@....com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Waiman Long <Waiman.Long@....com>
Subject: [PATCH v6 2/6] locking/pvqspinlock: Unconditional PV kick with _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 so is removed.

Signed-off-by: Waiman Long <Waiman.Long@....com>
Reviewed-by: Davidlohr Bueso <dave@...olabs.net>
---
 kernel/locking/qspinlock_paravirt.h |    6 +-----
 1 files changed, 1 insertions(+), 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
-- 
1.7.1

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