Re: [PATCH v3] locking/pvqspinlock: restore/set vcpu_hashed state after failing adaptive locking spinning

From: Waiman Long
Date: Thu Jul 14 2016 - 10:52:24 EST

On 07/14/2016 07:39 AM, Wanpeng Li wrote:
From: Wanpeng Li<>

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

CPU 0 (lock holder) CPU 1 (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);
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 which can result in hash entry leak.
This patch avoids it by restoring/setting vcpu_hashed state after
failing adaptive locking spinning.

Reviewed-by: Pan Xinhui<xinhui.pan@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra (Intel)<peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar<mingo@xxxxxxxxxx>
Cc: Waiman Long<Waiman.Long@xxxxxxx>
Cc: Davidlohr Bueso<dave@xxxxxxxxxxxx>
Signed-off-by: Wanpeng Li<>
v2 -> v3:
* fix typo in patch description
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);

As pv_kick_node() is called immediately after designating the next node as the queue head, the chance of this racing is possible, but is not likely unless the lock holder vCPU gets preempted for a long time at that right moment. This change does not do any harm though, so I am OK with that. However, I do want you to add a comment about the possible race in the code as it isn't that obvious or likely.