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

From: Davidlohr Bueso
Date: Sat Jul 16 2016 - 16:03:51 EST

On Sat, 16 Jul 2016, Wanpeng Li wrote:

From: Wanpeng Li <>

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():
lock->locked = _Q_SLOW_VAL;
node->state = vcpu_hashed;
vcpu_halted, vcpu_hashed);
lock->locked = _Q_SLOW_VAL;

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@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 <>

Reviewed-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>