[PATCH v4] locking/pvqspinlock: Fix double hash race

* [PATCH v4] locking/pvqspinlock: Fix double hash race
@ 2016-07-16  1:51 Wanpeng Li
  2016-07-16 20:03 ` Davidlohr Bueso
  2016-07-19  6:53 ` Wanpeng Li
  0 siblings, 2 replies; 4+ messages in thread
From: Wanpeng Li @ 2016-07-16  1:51 UTC (permalink / raw)
  To: linux-kernel, kvm
  Cc: Wanpeng Li, Peter Zijlstra (Intel),
	Ingo Molnar, Waiman Long, Davidlohr Bueso

From: Wanpeng Li <wanpeng.li@hotmail.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: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Waiman Long <Waiman.Long@hpe.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.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

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