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

[Davidlohr Bueso]

On Sat, 16 Jul 2016, Wanpeng Li wrote:

> 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: 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>

Reviewed-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>