Re: [PATCH] net: core: fix unbalanced qdisc_run_begin/qdisc_run_end

[Wen Yang <wenyang@linux.alibaba.com>]

Sorry, after analyzing the assembly code of this function,

the semantics of the short circuit, there is no problem in this place.

we will continue to analyze, please ignore this patch, thank you.

--

Regards,

Wen


On 2019/11/10 10:01 上午, Wen Yang wrote:
> 3598 static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
> 3599                                  struct net_device *dev,
> 3600                                  struct netdev_queue *txq)
> 3601 {
> ...
> 3650         } else if ((q->flags & TCQ_F_CAN_BYPASS) && !qdisc_qlen(q) &&
> 3651                    qdisc_run_begin(q)) {
>
> ---> Those multiple *and conditions* in this if statement are not
>       necessarily executed sequentially. If the qdisc_run_begin(q)
>       statement is executed first and the other conditions are not
>       satisfied, qdisc_run_end will have no chance to be executed,
>       and the lowest bit of q->running will always be 1.
>       This may lead to a softlockup:
>       https://bugzilla.kernel.org/show_bug.cgi?id=205427
> ...
> 3657
> 3658                 qdisc_bstats_update(q, skb);
> ...
> 3661                         if (unlikely(contended)) {
> 3662                                 spin_unlock(&q->busylock);
> 3663                                 contended = false;
> 3664                         }
> 3665                         __qdisc_run(q);
> 3666                 }
> 3667
> 3668                 qdisc_run_end(q);
> 3669                 rc = NET_XMIT_SUCCESS;
> 3670         }
> ...
>
> We ensure the correct execution order by explicitly
> specifying those dependencies.
> Fixes: edb09eb17ed8 ("net: sched: do not acquire qdisc spinlock in qdisc/class stats dump")
> Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Cong Wang <xiyou.wangcong@gmail.com>
> Cc: Jamal Hadi Salim <jhs@mojatatu.com>
> Cc: John Fastabend <john.fastabend@gmail.com>
> Cc: Kevin Athey <kda@google.com>
> Cc: Xiaotian Pei <xiaotian@google.com>
> Cc: netdev@vger.kernel.org
> Cc: bpf@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> ---
>   net/core/dev.c | 63 ++++++++++++++++++++++++++++++----------------------------
>   1 file changed, 33 insertions(+), 30 deletions(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 20c7a67..d2690ee 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3602,27 +3602,28 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
>   	spinlock_t *root_lock = qdisc_lock(q);
>   	struct sk_buff *to_free = NULL;
>   	bool contended;
> -	int rc;
> +	int rc = NET_XMIT_SUCCESS;
>   
>   	qdisc_calculate_pkt_len(skb, q);
>   
>   	if (q->flags & TCQ_F_NOLOCK) {
> -		if ((q->flags & TCQ_F_CAN_BYPASS) && q->empty &&
> -		    qdisc_run_begin(q)) {
> -			if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED,
> -					      &q->state))) {
> -				__qdisc_drop(skb, &to_free);
> -				rc = NET_XMIT_DROP;
> -				goto end_run;
> -			}
> -			qdisc_bstats_cpu_update(q, skb);
> +		if ((q->flags & TCQ_F_CAN_BYPASS) && q->empty) {
> +			if (qdisc_run_begin(q)) {
> +				if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED,
> +						      &q->state))) {
> +					__qdisc_drop(skb, &to_free);
> +					rc = NET_XMIT_DROP;
> +					goto end_run;
> +				}
> +				qdisc_bstats_cpu_update(q, skb);
>   
> -			rc = NET_XMIT_SUCCESS;
> -			if (sch_direct_xmit(skb, q, dev, txq, NULL, true))
> -				__qdisc_run(q);
> +				if (sch_direct_xmit(skb, q, dev, txq, NULL,
> +						    true))
> +					__qdisc_run(q);
>   
>   end_run:
> -			qdisc_run_end(q);
> +				qdisc_run_end(q);
> +			}
>   		} else {
>   			rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
>   			qdisc_run(q);
> @@ -3647,26 +3648,28 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
>   	if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {
>   		__qdisc_drop(skb, &to_free);
>   		rc = NET_XMIT_DROP;
> -	} else if ((q->flags & TCQ_F_CAN_BYPASS) && !qdisc_qlen(q) &&
> -		   qdisc_run_begin(q)) {
> -		/*
> -		 * This is a work-conserving queue; there are no old skbs
> -		 * waiting to be sent out; and the qdisc is not running -
> -		 * xmit the skb directly.
> -		 */
> +	} else if ((q->flags & TCQ_F_CAN_BYPASS) && !qdisc_qlen(q)) {
> +		if (qdisc_run_begin(q)) {
> +			/* This is a work-conserving queue;
> +			 * there are no old skbs waiting to be sent out;
> +			 * and the qdisc is not running -
> +			 * xmit the skb directly.
> +			 */
>   
> -		qdisc_bstats_update(q, skb);
> +			qdisc_bstats_update(q, skb);
>   
> -		if (sch_direct_xmit(skb, q, dev, txq, root_lock, true)) {
> -			if (unlikely(contended)) {
> -				spin_unlock(&q->busylock);
> -				contended = false;
> +			if (sch_direct_xmit(skb, q, dev, txq, root_lock,
> +					    true)) {
> +				if (unlikely(contended)) {
> +					spin_unlock(&q->busylock);
> +					contended = false;
> +				}
> +				__qdisc_run(q);
>   			}
> -			__qdisc_run(q);
> -		}
>   
> -		qdisc_run_end(q);
> -		rc = NET_XMIT_SUCCESS;
> +			qdisc_run_end(q);
> +			rc = NET_XMIT_SUCCESS;
> +		}
>   	} else {
>   		rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
>   		if (qdisc_run_begin(q)) {