Re: [PATCH v3 2/2] af_packet: fix soft lockup issue caused by tpacket_snd()

[luyun <luyun_611@163.com>]

在 2025/7/9 20:44, Eric Dumazet 写道:
> On Wed, Jul 9, 2025 at 2:57 AM Yun Lu <luyun_611@163.com> wrote:
>> From: Yun Lu <luyun@kylinos.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@kernel.org
>> Suggested-by: LongJun Tang <tanglongjun@kylinos.cn>
>> Signed-off-by: Yun Lu <luyun@kylinos.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;

Yes, the code of "else {} " is redundant. I will remove it in the next 
version.

Thanks.

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