linux-kernel - [PATCH v3 1/7] locking/pvqspinlock: Unconditional PV kick with _Q_SLOW

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Date:	Wed, 22 Jul 2015 16:12:36 -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 v3 1/7] locking/pvqspinlock: Unconditional PV kick with _Q_SLOW_VAL

The smp_store_release() is not a full barrier. In order to avoid missed
wakeup, we may need to add memory barrier around locked and cpu state
variables adding to complexity. As the chance of spurious wakeup is very
low, it is easier and safer to just do an unconditional kick at unlock
time.

Signed-off-by: Waiman Long <Waiman.Long@...com>
---
 kernel/locking/qspinlock_paravirt.h |    4 +---
 1 files changed, 1 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
index 15d3733..37bc363 100644
--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -240,7 +240,6 @@ static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node)
 			cpu_relax();
 		}
 
-		WRITE_ONCE(pn->state, vcpu_halted);
 		if (!lp) { /* ONCE */
 			lp = pv_hash(lock, pn);
 			/*
@@ -321,8 +320,7 @@ __visible void __pv_queued_spin_unlock(struct qspinlock *lock)
 	 * At this point the memory pointed at by lock can be freed/reused,
 	 * however we can still use the pv_node to kick the CPU.
 	 */
-	if (READ_ONCE(node->state) == vcpu_halted)
-		pv_kick(node->cpu);
+	pv_kick(node->cpu);
 }
 /*
  * Include the architecture specific callee-save thunk of the
-- 
1.7.1

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