linux-kernel - [PATCH v2] locking/pvqspinlock: restore/set vcpu_hashed state after failing adaptive locking spinning

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Message-Id: <1468484156-4521-1-git-send-email-wanpeng.li@hotmail.com>
Date:	Thu, 14 Jul 2016 16:15:56 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	linux-kernel@...r.kernel.org
Cc:	Wanpeng Li <wanpeng.li@...mail.com>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Waiman Long <Waiman.Long@....com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: [PATCH v2] locking/pvqspinlock: restore/set vcpu_hashed state after failing adaptive locking spinning

From: Wanpeng Li <wanpeng.li@...mail.com>

When the lock holder vCPU is racing with the queue head:

   CPU 0 (lock holder)    CPU1 (queue head)
   ===================    =================
   spin_lock();           spin_lock();
    pv_kick_node():        pv_wait_head_or_lock():
                            if (!lp) {
                             lp = pv_hash(lock, pn);
                             xchg(&l->locked, _Q_SLOW_VAL);
                            }
                            WRITE_ONCE(pn->state, vcpu_halted);
     cmpxchg(&pn->state, 
      vcpu_halted, vcpu_hashed);
     WRITE_ONCE(l->locked, _Q_SLOW_VAL);
     (void)pv_hash(lock, pn);

In this case, lock holder inserts the pv_node of queue head into the 
hash table and set _Q_SLOW_VAL unnecessary. This patch avoids it by 
restoring/setting vcpu_halted state after failing adaptive locking 
spinning.

Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Waiman Long <Waiman.Long@....com>
Cc: Davidlohr Bueso <dave@...olabs.net>
Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
---
v1 -> v2:
 * adjust patch description

 kernel/locking/qspinlock_paravirt.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
index 21ede57..ac7d20b 100644
--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -450,7 +450,7 @@ pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node)
 				goto gotlock;
 			}
 		}
-		WRITE_ONCE(pn->state, vcpu_halted);
+		WRITE_ONCE(pn->state, vcpu_hashed);
 		qstat_inc(qstat_pv_wait_head, true);
 		qstat_inc(qstat_pv_wait_again, waitcnt);
 		pv_wait(&l->locked, _Q_SLOW_VAL);
-- 
1.9.1