This is a note to let you know that I've just added the patch titled sfc: disable softirqs for ptp TX to the 5.18-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: sfc-disable-softirqs-for-ptp-tx.patch and it can be found in the queue-5.18 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit 8b1ab4a8751f1c86e1857d655def64a74f6328b6 Author: Alejandro Lucero <alejandro.lucero-palau@xxxxxxx> Date: Tue Jul 26 08:45:04 2022 +0200 sfc: disable softirqs for ptp TX [ Upstream commit 67c3b611d92fc238c43734878bc3e232ab570c79 ] Sending a PTP packet can imply to use the normal TX driver datapath but invoked from the driver's ptp worker. The kernel generic TX code disables softirqs and preemption before calling specific driver TX code, but the ptp worker does not. Although current ptp driver functionality does not require it, there are several reasons for doing so: 1) The invoked code is always executed with softirqs disabled for non PTP packets. 2) Better if a ptp packet transmission is not interrupted by softirq handling which could lead to high latencies. 3) netdev_xmit_more used by the TX code requires preemption to be disabled. Indeed a solution for dealing with kernel preemption state based on static kernel configuration is not possible since the introduction of dynamic preemption level configuration at boot time using the static calls functionality. Fixes: f79c957a0b537 ("drivers: net: sfc: use netdev_xmit_more helper") Signed-off-by: Alejandro Lucero <alejandro.lucero-palau@xxxxxxx> Link: https://lore.kernel.org/r/20220726064504.49613-1-alejandro.lucero-palau@xxxxxxx Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/drivers/net/ethernet/sfc/ptp.c b/drivers/net/ethernet/sfc/ptp.c index 4625f85acab2..10ad0b93d283 100644 --- a/drivers/net/ethernet/sfc/ptp.c +++ b/drivers/net/ethernet/sfc/ptp.c @@ -1100,7 +1100,29 @@ static void efx_ptp_xmit_skb_queue(struct efx_nic *efx, struct sk_buff *skb) tx_queue = efx_channel_get_tx_queue(ptp_data->channel, type); if (tx_queue && tx_queue->timestamping) { + /* This code invokes normal driver TX code which is always + * protected from softirqs when called from generic TX code, + * which in turn disables preemption. Look at __dev_queue_xmit + * which uses rcu_read_lock_bh disabling preemption for RCU + * plus disabling softirqs. We do not need RCU reader + * protection here. + * + * Although it is theoretically safe for current PTP TX/RX code + * running without disabling softirqs, there are three good + * reasond for doing so: + * + * 1) The code invoked is mainly implemented for non-PTP + * packets and it is always executed with softirqs + * disabled. + * 2) This being a single PTP packet, better to not + * interrupt its processing by softirqs which can lead + * to high latencies. + * 3) netdev_xmit_more checks preemption is disabled and + * triggers a BUG_ON if not. + */ + local_bh_disable(); efx_enqueue_skb(tx_queue, skb); + local_bh_enable(); } else { WARN_ONCE(1, "PTP channel has no timestamped tx queue\n"); dev_kfree_skb_any(skb);