Date: Wed, 22 May 2024 11:45:48 +0800
From: Heng Qi <hengqi@...ux.alibaba.com>
To: netdev@...r.kernel.org,
	virtualization@...ts.linux.dev
Cc: 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: [PATCH net 2/2] Revert "virtio_net: Add a lock for per queue RX coalesce"

This reverts commit 4d4ac2ececd3c42a08dd32a6e3a4aaf25f7efe44.

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>
---
 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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