While a new device is being added into fs to be sprouted, call stack is:
  btrfs_ioctl_add_dev
   btrfs_init_new_device
     btrfs_prepare_sprout
       list_splice_init_rcu(&fs_devices->devices, &seed_devices->devices,
       synchronize_rcu);
     list_add_rcu(&device->dev_list, &fs_devices->devices);

Looking at btrfs_prepare_sprout(), every new RCU reader will read a
empty fs_devices->devices once synchronize_rcu() is called.
After commit 4faf55b03823 ("btrfs: don't traverse into the seed devices
in show_devname"), btrfs_show_devname() won't looking into
fs_devices->seed_list even there is no device in fs_devices->devices.

And since commit 88c14590cdd6 ("btrfs: use RCU in btrfs_show_devname for
device list traversal"), btrfs_show_devname() only uses RCU without mutex
lock taken for device list traversal. The function read an empty
fs_devices->devices and found no device in the list then triggers the
warning. The commit just enlarged the window that the fs device list
could be empty. Even btrfs_show_devname() uses mutex_lock(), there is a
tiny chance to read an empty devices list between mutex_unlock() in
btrfs_prepare_sprout() and next mutex_lock() in btrfs_init_new_device().

So take device_list_mutex then traverse fs_devices->seed_list to seek
for a seed device if no device was found in fs_devices->devices.
Since a normal fs always has devices in fs_device->devices and the
window is small enough, the mutex lock is not too heavy.

Signed-off-by: Su Yue <redacted>

---
Changelog:
v2:
     Try to traverse fs_devices->seed_list instead of removing the
       WARN_ON().
     Change the subject.
     Add description of fix.
---
  fs/btrfs/super.c | 41 ++++++++++++++++++++++++++++++++++++++---
  1 file changed, 38 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index d07b18b2b250..31e723eb2ccf 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c

@@ -2482,7 +2482,9 @@ static int btrfs_unfreeze(struct super_block *sb)
  static int btrfs_show_devname(struct seq_file *m, struct dentry *root)
  {
  	struct btrfs_fs_info *fs_info = btrfs_sb(root->d_sb);
+	struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
  	struct btrfs_device *dev, *first_dev = NULL;
+	struct btrfs_fs_devices *seed_devices;
  
  	/*
  	 * Lightweight locking of the devices. We should not need

@@ -2492,7 +2494,7 @@ static int btrfs_show_devname(struct seq_file *m, struct dentry *root)
  	 * least until the rcu_read_unlock.
  	 */
  	rcu_read_lock();
-	list_for_each_entry_rcu(dev, &fs_info->fs_devices->devices, dev_list) {
+	list_for_each_entry_rcu(dev, &fs_devices->devices, dev_list) {
  		if (test_bit(BTRFS_DEV_STATE_MISSING, &dev->dev_state))
  			continue;
  		if (!dev->name)

@@ -2503,9 +2505,42 @@ static int btrfs_show_devname(struct seq_file *m, struct dentry *root)
  
  	if (first_dev)
  		seq_escape(m, rcu_str_deref(first_dev->name), " \t\n\\");
-	else
-		WARN_ON(1);
  	rcu_read_unlock();
+
+	if (first_dev)
+		return 0;
+
+	/*
+	 * While the fs is sprouting, above fs_devices->devices could be empty
+	 * if the RCU read happened in the window between when
+	 * fs_devices->devices was spliced into seed_devices->devices in
+	 * btrfs_prepare_sprout() and new device is not added to
+	 * fs_devices->devices in btrfs_init_new_device().
+	 *
+	 * Take device_list_mutex to make sure seed_devices has been added into
+	 * fs_devices->seed_list then we can traverse it.
+	 */
+	mutex_lock(&fs_devices->device_list_mutex);