From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
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@xxxxxxxxxxxx>
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 <wanpeng.li@xxxxxxxxxxx>
---
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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