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Message-ID: <CANn89iKF8=vT2vxYK1S=7mf6zqfTS5SVUYZG_BH1-6XLwiqV-g@mail.gmail.com>
Date: Wed, 9 Jul 2025 00:23:53 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: luyun <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 v2] af_packet: fix soft lockup issue caused by tpacket_snd()
On Tue, Jul 8, 2025 at 11:31 PM luyun <luyun_611@....com> wrote:
>
>
> 在 2025/7/8 15:12, Eric Dumazet 写道:
> > On Mon, Jul 7, 2025 at 7:06 PM 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, the
> >> execution condition of this function should be modified to check if
> >> pending_refcnt is not zero.
> >>
> >> Fixes: 89ed5b519004 ("af_packet: Block execution of tasks waiting for transmit to complete in AF_PACKET")
> >> Suggested-by: LongJun Tang <tanglongjun@...inos.cn>
> >> Signed-off-by: Yun Lu <luyun@...inos.cn>
> >>
> >> ---
> >> 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 | 2 +-
> >> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> >> index 3d43f3eae759..7df96311adb8 100644
> >> --- a/net/packet/af_packet.c
> >> +++ b/net/packet/af_packet.c
> >> @@ -2845,7 +2845,7 @@ 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) {
> >> + if (need_wait && packet_read_pending(&po->tx_ring)) {
> >> timeo = sock_sndtimeo(&po->sk, msg->msg_flags & MSG_DONTWAIT);
> >> timeo = wait_for_completion_interruptible_timeout(&po->skb_completion, timeo);
> >> if (timeo <= 0) {
> > packet_read_pending() is super expensive on hosts with 256 cpus (or more)
> >
> > We are going to call it a second time at the end of the block:
> >
> > do { ...
> > } while (ph != NULL || (need_wait && packet_read_pending(&po->tx_ring)...
> >
> > Perhaps we can remove the second one ?
>
> As mentioned in the commit message, the soft lockup issue only occurs
> when tpacket_snd() is called to send, with the pending_refcnt is
> non-zero, and there are no available packets in the tx_ring.
>
> Therefore, at the first start of the loop, packet_read_pending() only
> needs to be called once. If the return result is already 0, the loop can
> exit directly. Otherwise, wait_for_completion_interruptible_timeout()
> needs to be executed for waiting. Later, this function should only be
> called at the end to check whether the loop can exit.
>
>
> >
> > Also I think there is another problem with the code.
> >
> > We should call sock_sndtimeo() only once, otherwise SO_SNDTIMEO
> > constraint could be way off.
>
> Yes, due to the changes in commit 581073f626e3 ("af_packet: do not call
> packet_read_pending() from tpacket_destruct_skb()"), every time
> tpacket_destruct_skb is executed, the skb_completion is marked as
> completed. When wait_for_completion_interruptible_timeout returns
> completed, the pending_refcnt has not yet been reduced to 0. Therefore,
> when ph is NULL, the wait function may need to be called multiple times
> untill packet_read_pending finally returns 0.
>
>
> I have revised the code (as shown below), and it seems to be more
> reasonable. I also look forward to any better suggestions you may have.
> Thank you!
>
>
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index 7df96311adb8..401ae8f6481b 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -2785,7 +2785,9 @@ static int tpacket_snd(struct packet_sock *po,
> struct msghdr *msg)
> int len_sum = 0;
> int status = TP_STATUS_AVAILABLE;
> int hlen, tlen, copylen = 0;
> - long timeo = 0;
> + long timeo;
> + unsigned int pending;
> + bool first = true;
>
> mutex_lock(&po->pg_vec_lock);
>
> @@ -2839,18 +2841,27 @@ static int tpacket_snd(struct packet_sock *po,
> struct msghdr *msg)
> if ((size_max > dev->mtu + reserve + VLAN_HLEN) && !vnet_hdr_sz)
> size_max = dev->mtu + reserve + VLAN_HLEN;
>
> + timeo = sock_sndtimeo(&po->sk, msg->msg_flags & MSG_DONTWAIT);
> reinit_completion(&po->skb_completion);
>
> do {
> ph = packet_current_frame(po, &po->tx_ring,
> TP_STATUS_SEND_REQUEST);
> if (unlikely(ph == NULL)) {
> - if (need_wait &&
> packet_read_pending(&po->tx_ring)) {
> - timeo = sock_sndtimeo(&po->sk,
> msg->msg_flags & MSG_DONTWAIT);
> - timeo =
> wait_for_completion_interruptible_timeout(&po->skb_completion, timeo);
> - if (timeo <= 0) {
> - err = !timeo ? -ETIMEDOUT :
> -ERESTARTSYS;
> - goto out_put;
> + if (need_wait) {
> + if (skb == NULL && fisrt) {
> + pending =
> packet_read_pending(&po->tx_ring);
> + if (!pending)
> + goto out_put;
> + else
> + first = false;
> + }
> + if (skb || pending) {
> + timeo =
> wait_for_completion_interruptible_timeout(&po->skb_completion, timeo);
> + if (timeo <= 0) {
> + err = !timeo ?
> -ETIMEDOUT : -ERESTARTSYS;
> + goto out_put;
> + }
> }
> }
> /* check for additional frames */
>
>
> >
> > diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> > index f6b1ff883c9318facdcb9c3112b94f0b6e40d504..486ade64bddfddb1af91968dbdf70015cfb93eb5
> > 100644
> > --- a/net/packet/af_packet.c
> > +++ b/net/packet/af_packet.c
> > @@ -2785,8 +2785,9 @@ static int tpacket_snd(struct packet_sock *po,
> > struct msghdr *msg)
> > int len_sum = 0;
> > int status = TP_STATUS_AVAILABLE;
> > int hlen, tlen, copylen = 0;
> > - long timeo = 0;
> > + long timeo;
> >
> > + timeo = sock_sndtimeo(&po->sk, msg->msg_flags & MSG_DONTWAIT);
> > mutex_lock(&po->pg_vec_lock);
> >
> > /* packet_sendmsg() check on tx_ring.pg_vec was lockless,
> > @@ -2846,7 +2847,6 @@ static int tpacket_snd(struct packet_sock *po,
> > struct msghdr *msg)
> > TP_STATUS_SEND_REQUEST);
> > if (unlikely(ph == NULL)) {
> > if (need_wait && skb) {
> > - timeo = sock_sndtimeo(&po->sk,
> > msg->msg_flags & MSG_DONTWAIT);
> > timeo =
> > wait_for_completion_interruptible_timeout(&po->skb_completion, timeo);
> > if (timeo <= 0) {
> > err = !timeo ? -ETIMEDOUT :
> > -ERESTARTSYS;
>
Instead of adding two extra variables, you also could reuse ph (set it
to not zero)
Also please split in two different patches, one with the timeo fix alone.
Thanks !
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