6.12-stable review patch. If anyone has any objections, please let me know.
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From: Yun Lu luyun@kylinos.cn
commit 55f0bfc0370539213202f4ce1a07615327ac4713 upstream.
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, skb is true for all but the first iterations of that loop, and as long as pending_refcnt is not zero, even if incremented by a previous call, 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, instead of check skb.
- if (need_wait && skb) { + if (need_wait && packet_read_pending(&po->tx_ring)) {
As a result, the judgment conditions are duplicated with the end code of the while loop, and packet_read_pending() is a very expensive function. Actually, this loop can only exit when ph is NULL, so the loop condition can be changed to while (1), and in the "ph = NULL" branch, if the subsequent condition of if is not met, the loop can break directly. Now, the loop logic remains the same as origin but is clearer and more obvious.
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 Reviewed-by: Willem de Bruijn willemb@google.com Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/packet/af_packet.c | 23 +++++++++++------------ 1 file changed, 11 insertions(+), 12 deletions(-)
--- a/net/packet/af_packet.c +++ b/net/packet/af_packet.c @@ -2845,15 +2845,21 @@ static int tpacket_snd(struct packet_soc 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 the 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; } - } - /* check for additional frames */ - continue; + /* check for additional frames */ + continue; + } else + break; }
skb = NULL; @@ -2942,14 +2948,7 @@ tpacket_error: } packet_increment_head(&po->tx_ring); len_sum += tp_len; - } while (likely((ph != NULL) || - /* Note: packet_read_pending() might be slow if we have - * to call it as it's per_cpu variable, but in fast-path - * we already short-circuit the loop with the first - * condition, and luckily don't have to go that path - * anyway. - */ - (need_wait && packet_read_pending(&po->tx_ring)))); + } while (1);
err = len_sum; goto out_put;