Date: Wed, 22 May 2024 10:15:46 +0200
From: Jiri Pirko <jiri@...nulli.us>
To: Heng Qi <hengqi@...ux.alibaba.com>
Cc: netdev@...r.kernel.org, virtualization@...ts.linux.dev,
	Jason Wang <jasowang@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net 2/2] Revert "virtio_net: Add a lock for per queue RX
 coalesce"

Wed, May 22, 2024 at 05:45:48AM CEST, hengqi@...ux.alibaba.com wrote:
>This reverts commit 4d4ac2ececd3c42a08dd32a6e3a4aaf25f7efe44.

This commit does not exist in -net or -net-next.

>
>When the following snippet is run, lockdep will complain[1].
>
>  /* Acquire all queues dim_locks */
>  for (i = 0; i < vi->max_queue_pairs; i++)
>	  mutex_lock(&vi->rq[i].dim_lock);
>
>At the same time, too many queues will cause lockdep to be more irritable,
>which can be alleviated by using mutex_lock_nested(), however, there are
>still new warning when the number of queues exceeds MAX_LOCKDEP_SUBCLASSES.
>So I would like to gently revert this commit, although it brings
>unsynchronization that is not so concerned:
>  1. When dim is enabled, rx_dim_work may modify the coalescing parameters.
>     Users may read dirty coalescing parameters if querying.
>  2. When dim is switched from enabled to disabled, a spurious dim worker
>     maybe scheduled, but this can be handled correctly by rx_dim_work.
>
>[1]
>========================================================
>WARNING: possible recursive locking detected
>6.9.0-rc7+ #319 Not tainted
>--------------------------------------------
>ethtool/962 is trying to acquire lock:
>
>but task is already holding lock:
>
>other info that might help us debug this:
>Possible unsafe locking scenario:
>
>      CPU0
>      ----
> lock(&vi->rq[i].dim_lock);
> lock(&vi->rq[i].dim_lock);
>
>*** DEADLOCK ***
>
> May be due to missing lock nesting notation
>
>3 locks held by ethtool/962:
> #0: ffffffff82dbaab0 (cb_lock){++++}-{3:3}, at: genl_rcv+0x19/0x40
> #1: ffffffff82dad0a8 (rtnl_mutex){+.+.}-{3:3}, at:
>				ethnl_default_set_doit+0xbe/0x1e0
>
>stack backtrace:
>CPU: 6 PID: 962 Comm: ethtool Not tainted 6.9.0-rc7+ #319
>Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
>	   rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
>Call Trace:
> <TASK>
> dump_stack_lvl+0x79/0xb0
> check_deadlock+0x130/0x220
> __lock_acquire+0x861/0x990
> lock_acquire.part.0+0x72/0x1d0
> ? lock_acquire+0xf8/0x130
> __mutex_lock+0x71/0xd50
> virtnet_set_coalesce+0x151/0x190
> __ethnl_set_coalesce.isra.0+0x3f8/0x4d0
> ethnl_set_coalesce+0x34/0x90
> ethnl_default_set_doit+0xdd/0x1e0
> genl_family_rcv_msg_doit+0xdc/0x130
> genl_family_rcv_msg+0x154/0x230
> ? __pfx_ethnl_default_set_doit+0x10/0x10
> genl_rcv_msg+0x4b/0xa0
> ? __pfx_genl_rcv_msg+0x10/0x10
> netlink_rcv_skb+0x5a/0x110
> genl_rcv+0x28/0x40
> netlink_unicast+0x1af/0x280
> netlink_sendmsg+0x20e/0x460
> __sys_sendto+0x1fe/0x210
> ? find_held_lock+0x2b/0x80
> ? do_user_addr_fault+0x3a2/0x8a0
> ? __lock_release+0x5e/0x160
> ? do_user_addr_fault+0x3a2/0x8a0
> ? lock_release+0x72/0x140
> ? do_user_addr_fault+0x3a7/0x8a0
> __x64_sys_sendto+0x29/0x30
> do_syscall_64+0x78/0x180
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
>
>Signed-off-by: Heng Qi <hengqi@...ux.alibaba.com>

Fixes tag missing.


>---
> drivers/net/virtio_net.c | 53 +++++++++-------------------------------
> 1 file changed, 12 insertions(+), 41 deletions(-)
>
>diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>index 1cad06cef230..e4a1dff2a64a 100644
>--- a/drivers/net/virtio_net.c
>+++ b/drivers/net/virtio_net.c
>@@ -316,9 +316,6 @@ struct receive_queue {
> 	/* Is dynamic interrupt moderation enabled? */
> 	bool dim_enabled;
> 
>-	/* Used to protect dim_enabled and inter_coal */
>-	struct mutex dim_lock;
>-
> 	/* Dynamic Interrupt Moderation */
> 	struct dim dim;
> 
>@@ -2368,10 +2365,6 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
> 	/* Out of packets? */
> 	if (received < budget) {
> 		napi_complete = virtqueue_napi_complete(napi, rq->vq, received);
>-		/* Intentionally not taking dim_lock here. This may result in a
>-		 * spurious net_dim call. But if that happens virtnet_rx_dim_work
>-		 * will not act on the scheduled work.
>-		 */
> 		if (napi_complete && rq->dim_enabled)
> 			virtnet_rx_dim_update(vi, rq);
> 	}
>@@ -3247,11 +3240,9 @@ static int virtnet_set_ringparam(struct net_device *dev,
> 				return err;
> 
> 			/* The reason is same as the transmit virtqueue reset */
>-			mutex_lock(&vi->rq[i].dim_lock);
> 			err = virtnet_send_rx_ctrl_coal_vq_cmd(vi, i,
> 							       vi->intr_coal_rx.max_usecs,
> 							       vi->intr_coal_rx.max_packets);
>-			mutex_unlock(&vi->rq[i].dim_lock);
> 			if (err)
> 				return err;
> 		}
>@@ -4257,7 +4248,6 @@ static int virtnet_send_rx_notf_coal_cmds(struct virtnet_info *vi,
> 	struct virtio_net_ctrl_coal_rx *coal_rx __free(kfree) = NULL;
> 	bool rx_ctrl_dim_on = !!ec->use_adaptive_rx_coalesce;
> 	struct scatterlist sgs_rx;
>-	int ret = 0;
> 	int i;
> 
> 	if (rx_ctrl_dim_on && !virtio_has_feature(vi->vdev, VIRTIO_NET_F_VQ_NOTF_COAL))
>@@ -4267,22 +4257,16 @@ static int virtnet_send_rx_notf_coal_cmds(struct virtnet_info *vi,
> 			       ec->rx_max_coalesced_frames != vi->intr_coal_rx.max_packets))
> 		return -EINVAL;
> 
>-	/* Acquire all queues dim_locks */
>-	for (i = 0; i < vi->max_queue_pairs; i++)
>-		mutex_lock(&vi->rq[i].dim_lock);
>-
> 	if (rx_ctrl_dim_on && !vi->rx_dim_enabled) {
> 		vi->rx_dim_enabled = true;
> 		for (i = 0; i < vi->max_queue_pairs; i++)
> 			vi->rq[i].dim_enabled = true;
>-		goto unlock;
>+		return 0;
> 	}
> 
> 	coal_rx = kzalloc(sizeof(*coal_rx), GFP_KERNEL);
>-	if (!coal_rx) {
>-		ret = -ENOMEM;
>-		goto unlock;
>-	}
>+	if (!coal_rx)
>+		return -ENOMEM;
> 
> 	if (!rx_ctrl_dim_on && vi->rx_dim_enabled) {
> 		vi->rx_dim_enabled = false;
>@@ -4300,10 +4284,8 @@ static int virtnet_send_rx_notf_coal_cmds(struct virtnet_info *vi,
> 
> 	if (!virtnet_send_command(vi, VIRTIO_NET_CTRL_NOTF_COAL,
> 				  VIRTIO_NET_CTRL_NOTF_COAL_RX_SET,
>-				  &sgs_rx)) {
>-		ret = -EINVAL;
>-		goto unlock;
>-	}
>+				  &sgs_rx))
>+		return -EINVAL;
> 
> 	vi->intr_coal_rx.max_usecs = ec->rx_coalesce_usecs;
> 	vi->intr_coal_rx.max_packets = ec->rx_max_coalesced_frames;
>@@ -4311,11 +4293,8 @@ static int virtnet_send_rx_notf_coal_cmds(struct virtnet_info *vi,
> 		vi->rq[i].intr_coal.max_usecs = ec->rx_coalesce_usecs;
> 		vi->rq[i].intr_coal.max_packets = ec->rx_max_coalesced_frames;
> 	}
>-unlock:
>-	for (i = vi->max_queue_pairs - 1; i >= 0; i--)
>-		mutex_unlock(&vi->rq[i].dim_lock);
> 
>-	return ret;
>+	return 0;
> }
> 
> static int virtnet_send_notf_coal_cmds(struct virtnet_info *vi,
>@@ -4339,24 +4318,19 @@ static int virtnet_send_rx_notf_coal_vq_cmds(struct virtnet_info *vi,
> 					     u16 queue)
> {
> 	bool rx_ctrl_dim_on = !!ec->use_adaptive_rx_coalesce;
>+	bool cur_rx_dim = vi->rq[queue].dim_enabled;
> 	u32 max_usecs, max_packets;
>-	bool cur_rx_dim;
> 	int err;
> 
>-	mutex_lock(&vi->rq[queue].dim_lock);
>-	cur_rx_dim = vi->rq[queue].dim_enabled;
> 	max_usecs = vi->rq[queue].intr_coal.max_usecs;
> 	max_packets = vi->rq[queue].intr_coal.max_packets;
> 
> 	if (rx_ctrl_dim_on && (ec->rx_coalesce_usecs != max_usecs ||
>-			       ec->rx_max_coalesced_frames != max_packets)) {
>-		mutex_unlock(&vi->rq[queue].dim_lock);
>+			       ec->rx_max_coalesced_frames != max_packets))
> 		return -EINVAL;
>-	}
> 
> 	if (rx_ctrl_dim_on && !cur_rx_dim) {
> 		vi->rq[queue].dim_enabled = true;
>-		mutex_unlock(&vi->rq[queue].dim_lock);
> 		return 0;
> 	}
> 
>@@ -4369,8 +4343,10 @@ static int virtnet_send_rx_notf_coal_vq_cmds(struct virtnet_info *vi,
> 	err = virtnet_send_rx_ctrl_coal_vq_cmd(vi, queue,
> 					       ec->rx_coalesce_usecs,
> 					       ec->rx_max_coalesced_frames);
>-	mutex_unlock(&vi->rq[queue].dim_lock);
>-	return err;
>+	if (err)
>+		return err;
>+
>+	return 0;
> }
> 
> static int virtnet_send_notf_coal_vq_cmds(struct virtnet_info *vi,
>@@ -4404,7 +4380,6 @@ static void virtnet_rx_dim_work(struct work_struct *work)
> 
> 	qnum = rq - vi->rq;
> 
>-	mutex_lock(&rq->dim_lock);
> 	if (!rq->dim_enabled)
> 		goto out;
> 
>@@ -4420,7 +4395,6 @@ static void virtnet_rx_dim_work(struct work_struct *work)
> 	}
> out:
> 	dim->state = DIM_START_MEASURE;
>-	mutex_unlock(&rq->dim_lock);
> }
> 
> static int virtnet_coal_params_supported(struct ethtool_coalesce *ec)
>@@ -4558,13 +4532,11 @@ static int virtnet_get_per_queue_coalesce(struct net_device *dev,
> 		return -EINVAL;
> 
> 	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_VQ_NOTF_COAL)) {
>-		mutex_lock(&vi->rq[queue].dim_lock);
> 		ec->rx_coalesce_usecs = vi->rq[queue].intr_coal.max_usecs;
> 		ec->tx_coalesce_usecs = vi->sq[queue].intr_coal.max_usecs;
> 		ec->tx_max_coalesced_frames = vi->sq[queue].intr_coal.max_packets;
> 		ec->rx_max_coalesced_frames = vi->rq[queue].intr_coal.max_packets;
> 		ec->use_adaptive_rx_coalesce = vi->rq[queue].dim_enabled;
>-		mutex_unlock(&vi->rq[queue].dim_lock);
> 	} else {
> 		ec->rx_max_coalesced_frames = 1;
> 
>@@ -5396,7 +5368,6 @@ static int virtnet_alloc_queues(struct virtnet_info *vi)
> 
> 		u64_stats_init(&vi->rq[i].stats.syncp);
> 		u64_stats_init(&vi->sq[i].stats.syncp);
>-		mutex_init(&vi->rq[i].dim_lock);
> 	}
> 
> 	return 0;
>-- 
>2.32.0.3.g01195cf9f
>
>

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