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

[Waiman Long <waiman.long@hpe.com>]

On 07/27/2016 07:30 AM, Wanpeng Li wrote:
> From: Wanpeng Li<wanpeng.li@hotmail.com>
>
> 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@stgolabs.net>
> Reviewed-by: Pan Xinhui<xinhui.pan@linux.vnet.ibm.com>
> Cc: Peter Zijlstra (Intel)<peterz@infradead.org>
> Cc: Ingo Molnar<mingo@kernel.org>
> Cc: Waiman Long<Waiman.Long@hpe.com>
> Cc: Davidlohr Bueso<dave@stgolabs.net>
> Signed-off-by: Wanpeng Li<wanpeng.li@hotmail.com>
> ---
> 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);

Acked-by: Waiman Long <Waiman.Long@hpe.com>

Cheers,
Longman