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

From: Wanpeng Li
Date: Tue Jul 19 2016 - 02:53:25 EST


Sorry for the quick ping, but could we catch the merge window? Ingo. :)
2016-07-16 9:51 GMT+08:00 Wanpeng Li <kernellwp@xxxxxxxxx>:
> 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>
> ---
> 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
>



--
Regards,
Wanpeng Li