linux-kernel - [PATCH RESEND v4] locking/pvqspinlock: Fix double hash race

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Message-Id: <1469619037-13826-1-git-send-email-wanpeng.li@hotmail.com>
Date:	Wed, 27 Jul 2016 19:30:37 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:	Wanpeng Li <wanpeng.li@...mail.com>,
	Waiman Long <Waiman.Long@....com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: [PATCH RESEND v4] locking/pvqspinlock: Fix double hash race

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

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

lock holder vCPU             queue head vCPU
=====================        ==================

node->locked = 1;
<preemption>                 READ_ONCE(node->locked)
   ...                       pv_wait_head_or_lock():
                               SPIN_THRESHOLD loop;
                               pv_hash();
                               lock->locked = _Q_SLOW_VAL;
                               node->state  = vcpu_hashed;
pv_kick_node():
  cmpxchg(node->state,
     vcpu_halted, vcpu_hashed);
  lock->locked = _Q_SLOW_VAL;
  pv_hash();

With preemption at the right moment, it is possible that both the
lock holder and queue head vCPUs can be racing to set node->state
which can result in hash entry race. Making sure the state is never 
set to vcpu_halted will prevent this racing from happening.
 
This patch fix it by setting vcpu_hashed after we did all hash thing.

Reviewed-by: Davidlohr Bueso <dave@...olabs.net>
Reviewed-by: Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>
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>
---
v3 -> v4:
 * update patch subject
 * add code comments
v2 -> v3:
 * fix typo in patch description
v1 -> v2:
 * adjust patch description

 kernel/locking/qspinlock_paravirt.h | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
index 21ede57..ca96db4 100644
--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -450,7 +450,28 @@ pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node)
 				goto gotlock;
 			}
 		}
-		WRITE_ONCE(pn->state, vcpu_halted);
+		/*
+		 * lock holder vCPU             queue head vCPU
+		 * ----------------             ---------------
+		 * node->locked = 1;
+		 * <preemption>                 READ_ONCE(node->locked)
+		 *    ...                       pv_wait_head_or_lock():
+		 *                                SPIN_THRESHOLD loop;
+		 *                                pv_hash();
+		 *                                lock->locked = _Q_SLOW_VAL;
+		 *                                node->state  = vcpu_hashed;
+		 * pv_kick_node():
+		 *   cmpxchg(node->state,
+		 *      vcpu_halted, vcpu_hashed);
+		 *   lock->locked = _Q_SLOW_VAL;
+		 *   pv_hash();
+		 *
+		 * With preemption at the right moment, it is possible that both the
+		 * lock holder and queue head vCPUs can be racing to set node->state.
+		 * Making sure the state is never set to vcpu_halted will prevent this
+		 * racing from happening.
+		 */
+		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);
-- 
2.1.0