md/cluster: fix deadlock when node is doing resync job

md-cluster uses MD_CLUSTER_SEND_LOCK to make node can exclusively send msg.
During sending msg, node can concurrently receive msg from another node.
When node does resync job, grab token_lockres:EX may trigger a deadlock:
```
nodeA nodeB
-------------------- --------------------
a.
send METADATA_UPDATED
held token_lockres:EX
b.
md_do_sync
resync_info_update
send RESYNCING
+ set MD_CLUSTER_SEND_LOCK
+ wait for holding token_lockres:EX

c.
mdadm /dev/md0 --remove /dev/sdg
+ held reconfig_mutex
+ send REMOVE
+ wait_event(MD_CLUSTER_SEND_LOCK)

d.
recv_daemon //METADATA_UPDATED from A
process_metadata_update
+ (mddev_trylock(mddev) ||
MD_CLUSTER_HOLDING_MUTEX_FOR_RECVD)
//this time, both return false forever
```
Explaination:
a. A send METADATA_UPDATED
This will block another node to send msg

b. B does sync jobs, which will send RESYNCING at intervals.
This will be block for holding token_lockres:EX lock.

c. B do "mdadm --remove", which will send REMOVE.
This will be blocked by step <b>: MD_CLUSTER_SEND_LOCK is 1.

d. B recv METADATA_UPDATED msg, which send from A in step <a>.
This will be blocked by step <c>: holding mddev lock, it makes
wait_event can't hold mddev lock. (btw,
MD_CLUSTER_HOLDING_MUTEX_FOR_RECVD keep ZERO in this scenario.)

There is a similar deadlock in commit 0ba959774e93
("md-cluster: use sync way to handle METADATA_UPDATED msg")
In that commit, step c is "update sb". This patch step c is
"mdadm --remove".

For fixing this issue, we can refer the solution of function:
metadata_update_start. Which does the same grab lock_token action.
lock_comm can use the same steps to avoid deadlock. By moving
MD_CLUSTER_HOLDING_MUTEX_FOR_RECVD from lock_token to lock_comm.
It enlarge a little bit window of MD_CLUSTER_HOLDING_MUTEX_FOR_RECVD,
but it is safe & can break deadlock.

Repro steps (I only triggered 3 times with hundreds tests):

two nodes share 3 iSCSI luns: sdg/sdh/sdi. Each lun size is 1GB.
```
ssh root@node2 "mdadm -S --scan"
mdadm -S --scan
for i in {g,h,i};do dd if=/dev/zero of=/dev/sd$i oflag=direct bs=1M \
count=20; done

mdadm -C /dev/md0 -b clustered -e 1.2 -n 2 -l mirror /dev/sdg /dev/sdh \
--bitmap-chunk=1M
ssh root@node2 "mdadm -A /dev/md0 /dev/sdg /dev/sdh"

sleep 5

mkfs.xfs /dev/md0
mdadm --manage --add /dev/md0 /dev/sdi
mdadm --wait /dev/md0
mdadm --grow --raid-devices=3 /dev/md0

mdadm /dev/md0 --fail /dev/sdg
mdadm /dev/md0 --remove /dev/sdg
mdadm --grow --raid-devices=2 /dev/md0
```

test script will hung when executing "mdadm --remove".

```
# dump stacks by "echo t > /proc/sysrq-trigger"
md0_cluster_rec D 0 5329 2 0x80004000
Call Trace:
__schedule+0x1f6/0x560
? _cond_resched+0x2d/0x40
? schedule+0x4a/0xb0
? process_metadata_update.isra.0+0xdb/0x140 [md_cluster]
? wait_woken+0x80/0x80
? process_recvd_msg+0x113/0x1d0 [md_cluster]
? recv_daemon+0x9e/0x120 [md_cluster]
? md_thread+0x94/0x160 [md_mod]
? wait_woken+0x80/0x80
? md_congested+0x30/0x30 [md_mod]
? kthread+0x115/0x140
? __kthread_bind_mask+0x60/0x60
? ret_from_fork+0x1f/0x40

mdadm D 0 5423 1 0x00004004
Call Trace:
__schedule+0x1f6/0x560
? __schedule+0x1fe/0x560
? schedule+0x4a/0xb0
? lock_comm.isra.0+0x7b/0xb0 [md_cluster]
? wait_woken+0x80/0x80
? remove_disk+0x4f/0x90 [md_cluster]
? hot_remove_disk+0xb1/0x1b0 [md_mod]
? md_ioctl+0x50c/0xba0 [md_mod]
? wait_woken+0x80/0x80
? blkdev_ioctl+0xa2/0x2a0
? block_ioctl+0x39/0x40
? ksys_ioctl+0x82/0xc0
? __x64_sys_ioctl+0x16/0x20
? do_syscall_64+0x5f/0x150
? entry_SYSCALL_64_after_hwframe+0x44/0xa9

md0_resync D 0 5425 2 0x80004000
Call Trace:
__schedule+0x1f6/0x560
? schedule+0x4a/0xb0
? dlm_lock_sync+0xa1/0xd0 [md_cluster]
? wait_woken+0x80/0x80
? lock_token+0x2d/0x90 [md_cluster]
? resync_info_update+0x95/0x100 [md_cluster]
? raid1_sync_request+0x7d3/0xa40 [raid1]
? md_do_sync.cold+0x737/0xc8f [md_mod]
? md_thread+0x94/0x160 [md_mod]
? md_congested+0x30/0x30 [md_mod]
? kthread+0x115/0x140
? __kthread_bind_mask+0x60/0x60
? ret_from_fork+0x1f/0x40
```

At last, thanks for Xiao's solution.

Cc: [e-mail]
Signed-off-by: Zhao Heming <[e-mail]>
Suggested-by: Xiao Ni <[e-mail]>
Reviewed-by: Xiao Ni <[e-mail]>
Signed-off-by: Song Liu <[e-mail]>

drivers/md/md-cluster.c

···
  * Takes the lock on the TOKEN lock resource so no other
  * node can communicate while the operation is underway.
  */
-static int lock_token(struct md_cluster_info *cinfo, bool mddev_locked)
+static int lock_token(struct md_cluster_info *cinfo)
 {
-	int error, set_bit = 0;
+	int error;
+
+	error = dlm_lock_sync(cinfo->token_lockres, DLM_LOCK_EX);
+	if (error) {
+		pr_err("md-cluster(%s:%d): failed to get EX on TOKEN (%d)\n",
+				__func__, __LINE__, error);
+	} else {
+		/* Lock the receive sequence */
+		mutex_lock(&cinfo->recv_mutex);
+	}
+	return error;
+}
+
+/* lock_comm()
+ * Sets the MD_CLUSTER_SEND_LOCK bit to lock the send channel.
+ */
+static int lock_comm(struct md_cluster_info *cinfo, bool mddev_locked)
+{
+	int rv, set_bit = 0;
 	struct mddev *mddev = cinfo->mddev;
 
 	/*
···
 	 */
 	if (mddev_locked && !test_bit(MD_CLUSTER_HOLDING_MUTEX_FOR_RECVD,
 				      &cinfo->state)) {
-		error = test_and_set_bit_lock(MD_CLUSTER_HOLDING_MUTEX_FOR_RECVD,
+		rv = test_and_set_bit_lock(MD_CLUSTER_HOLDING_MUTEX_FOR_RECVD,
 					      &cinfo->state);
-		WARN_ON_ONCE(error);
+		WARN_ON_ONCE(rv);
 		md_wakeup_thread(mddev->thread);
 		set_bit = 1;
 	}
-	error = dlm_lock_sync(cinfo->token_lockres, DLM_LOCK_EX);
-	if (set_bit)
-		clear_bit_unlock(MD_CLUSTER_HOLDING_MUTEX_FOR_RECVD, &cinfo->state);
 
-	if (error)
-		pr_err("md-cluster(%s:%d): failed to get EX on TOKEN (%d)\n",
-				__func__, __LINE__, error);
-
-	/* Lock the receive sequence */
-	mutex_lock(&cinfo->recv_mutex);
-	return error;
-}
-
-/* lock_comm()
- * Sets the MD_CLUSTER_SEND_LOCK bit to lock the send channel.
- */
-static int lock_comm(struct md_cluster_info *cinfo, bool mddev_locked)
-{
 	wait_event(cinfo->wait,
 		   !test_and_set_bit(MD_CLUSTER_SEND_LOCK, &cinfo->state));
-
-	return lock_token(cinfo, mddev_locked);
+	rv = lock_token(cinfo);
+	if (set_bit)
+		clear_bit_unlock(MD_CLUSTER_HOLDING_MUTEX_FOR_RECVD, &cinfo->state);
+	return rv;
 }
 
 static void unlock_comm(struct md_cluster_info *cinfo)
···
 {
 	int ret;
 
-	lock_comm(cinfo, mddev_locked);
-	ret = __sendmsg(cinfo, cmsg);
-	unlock_comm(cinfo);
+	ret = lock_comm(cinfo, mddev_locked);
+	if (!ret) {
+		ret = __sendmsg(cinfo, cmsg);
+		unlock_comm(cinfo);
+	}
 	return ret;
 }
 
···
 		return 0;
 	}
 
-	ret = lock_token(cinfo, 1);
+	ret = lock_token(cinfo);
 	clear_bit_unlock(MD_CLUSTER_HOLDING_MUTEX_FOR_RECVD, &cinfo->state);
 	return ret;
 }
···
 	int raid_slot = -1;
 
 	md_update_sb(mddev, 1);
-	lock_comm(cinfo, 1);
+	if (lock_comm(cinfo, 1)) {
+		pr_err("%s: lock_comm failed\n", __func__);
+		return;
+	}
 
 	memset(&cmsg, 0, sizeof(cmsg));
 	cmsg.type = cpu_to_le32(METADATA_UPDATED);
···
 	cmsg.type = cpu_to_le32(NEWDISK);
 	memcpy(cmsg.uuid, uuid, 16);
 	cmsg.raid_slot = cpu_to_le32(rdev->desc_nr);
-	lock_comm(cinfo, 1);
+	if (lock_comm(cinfo, 1))
+		return -EAGAIN;
 	ret = __sendmsg(cinfo, &cmsg);
 	if (ret) {
 		unlock_comm(cinfo);

drivers/md/md.c

···
 		goto busy;
 
 kick_rdev:
-	if (mddev_is_clustered(mddev))
-		md_cluster_ops->remove_disk(mddev, rdev);
+	if (mddev_is_clustered(mddev)) {
+		if (md_cluster_ops->remove_disk(mddev, rdev))
+			goto busy;
+	}
 
 	md_kick_rdev_from_array(rdev);
 	set_bit(MD_SB_CHANGE_DEVS, &mddev->sb_flags);