virtio_net: fix a spurious deadlock issue

When the following snippet is run, lockdep will report a deadlock[1].

  /* Acquire all queues dim_locks */
  for (i = 0; i < vi->max_queue_pairs; i++)
          mutex_lock(&vi->rq[i].dim_lock);

There's no deadlock here because the vq locks are always taken
in the same order, but lockdep can not figure it out. So refactoring
the code to alleviate the problem.

[1]
========================================================
WARNING: possible recursive locking detected
6.9.0-rc7+ torvalds#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+ torvalds#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

Fixes: 4d4ac2e ("virtio_net: Add a lock for per queue RX coalesce")
Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20240528134116.117426-3-hengqi@linux.alibaba.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/virtio_net.c

@@ -4259,7 +4259,6 @@ 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))
@@ -4269,27 +4268,27 @@ 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++)
+		for (i = 0; i < vi->max_queue_pairs; i++) {
+			mutex_lock(&vi->rq[i].dim_lock);
 			vi->rq[i].dim_enabled = true;
-		goto unlock;
+			mutex_unlock(&vi->rq[i].dim_lock);
+		}
+		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;
-		for (i = 0; i < vi->max_queue_pairs; i++)
+		for (i = 0; i < vi->max_queue_pairs; i++) {
+			mutex_lock(&vi->rq[i].dim_lock);
 			vi->rq[i].dim_enabled = false;
+			mutex_unlock(&vi->rq[i].dim_lock);
+		}
 	}
 
 	/* Since the per-queue coalescing params can be set,
@@ -4302,22 +4301,19 @@ 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;
 	for (i = 0; i < vi->max_queue_pairs; i++) {
+		mutex_lock(&vi->rq[i].dim_lock);
 		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,