linux-kernel - Re: [PATCH v3 2/2] af_packet: fix soft lockup issue caused by tpacket

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Message-ID: <CANn89iJZ=t6Fg4fjgPooyTAbD4Lxj9AKFQx_mnJty5nq9Ng9vw@mail.gmail.com>
Date: Wed, 9 Jul 2025 05:44:14 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Yun Lu <luyun_611@....com>
Cc: willemdebruijn.kernel@...il.com, davem@...emloft.net, kuba@...nel.org, 
	pabeni@...hat.com, netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] af_packet: fix soft lockup issue caused by tpacket_snd()

On Wed, Jul 9, 2025 at 2:57 AM Yun Lu <luyun_611@....com> wrote:
>
> From: Yun Lu <luyun@...inos.cn>
>
> When MSG_DONTWAIT is not set, the tpacket_snd operation will wait for
> pending_refcnt to decrement to zero before returning. The pending_refcnt
> is decremented by 1 when the skb->destructor function is called,
> indicating that the skb has been successfully sent and needs to be
> destroyed.
>
> If an error occurs during this process, the tpacket_snd() function will
> exit and return error, but pending_refcnt may not yet have decremented to
> zero. Assuming the next send operation is executed immediately, but there
> are no available frames to be sent in tx_ring (i.e., packet_current_frame
> returns NULL), and skb is also NULL, the function will not execute
> wait_for_completion_interruptible_timeout() to yield the CPU. Instead, it
> will enter a do-while loop, waiting for pending_refcnt to be zero. Even
> if the previous skb has completed transmission, the skb->destructor
> function can only be invoked in the ksoftirqd thread (assuming NAPI
> threading is enabled). When both the ksoftirqd thread and the tpacket_snd
> operation happen to run on the same CPU, and the CPU trapped in the
> do-while loop without yielding, the ksoftirqd thread will not get
> scheduled to run. As a result, pending_refcnt will never be reduced to
> zero, and the do-while loop cannot exit, eventually leading to a CPU soft
> lockup issue.
>
> In fact, as long as pending_refcnt is not zero, even if skb is NULL,
> wait_for_completion_interruptible_timeout() should be executed to yield
> the CPU, allowing the ksoftirqd thread to be scheduled. Therefore, move
> the penging_refcnt check to the start of the do-while loop, and reuse ph
> to continue for the next iteration.
>
> Fixes: 89ed5b519004 ("af_packet: Block execution of tasks waiting for transmit to complete in AF_PACKET")
> Cc: stable@...nel.org
> Suggested-by: LongJun Tang <tanglongjun@...inos.cn>
> Signed-off-by: Yun Lu <luyun@...inos.cn>
>
> ---
> Changes in v3:
> - Simplify the code and reuse ph to continue. Thanks: Eric Dumazet.
> - Link to v2: https://lore.kernel.org/all/20250708020642.27838-1-luyun_611@163.com/
>
> Changes in v2:
> - Add a Fixes tag.
> - Link to v1: https://lore.kernel.org/all/20250707081629.10344-1-luyun_611@163.com/
> ---
>  net/packet/af_packet.c | 21 ++++++++++++---------
>  1 file changed, 12 insertions(+), 9 deletions(-)
>
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index 7089b8c2a655..89a5d2a3a720 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -2846,11 +2846,21 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
>                 ph = packet_current_frame(po, &po->tx_ring,
>                                           TP_STATUS_SEND_REQUEST);
>                 if (unlikely(ph == NULL)) {
> -                       if (need_wait && skb) {
> +                       /* Note: packet_read_pending() might be slow if we
> +                        * have to call it as it's per_cpu variable, but in
> +                        * fast-path we don't have to call it, only when ph
> +                        * is NULL, we need to check pending_refcnt.
> +                        */
> +                       if (need_wait && packet_read_pending(&po->tx_ring)) {
>                                 timeo = wait_for_completion_interruptible_timeout(&po->skb_completion, timeo);
>                                 if (timeo <= 0) {
>                                         err = !timeo ? -ETIMEDOUT : -ERESTARTSYS;
>                                         goto out_put;
> +                               } else {

nit (in case a new version is sent) : No need for an else {} after a
"goto XXXXX;"

if (....) {
     .....
     goto out_put;
}
/* Just reuse ph to continue for the next iteration, and...
 * .....
 */
ph = (void *)1;


Reviewed-by: Eric Dumazet <edumazet@...gle.com>