[PATCH] btrfs: fix warn_on for send from readonly mount

[Anand Jain <anand.jain@oracle.com>]

We log warning if root::orphan_cleanup_state is not set to
ORPHAN_CLEANUP_DONE in btrfs_ioctl_send(). However if the filesystem is
mounted as readonly we skip the orphan items cleanup during the lookup
and root::orphan_cleanup_state remains at the init state 0 instead of
ORPHAN_CLEANUP_DONE (2).

WARNING: CPU: 0 PID: 2616 at /Volumes/ws/btrfs-devel/fs/btrfs/send.c:7090 btrfs_ioctl_send+0xb2f/0x18c0 [btrfs]
::
RIP: 0010:btrfs_ioctl_send+0xb2f/0x18c0 [btrfs]
::
Call Trace:
::
_btrfs_ioctl_send+0x7b/0x110 [btrfs]
btrfs_ioctl+0x150a/0x2b00 [btrfs]
::
do_vfs_ioctl+0xa9/0x620
? __fget+0xac/0xe0
ksys_ioctl+0x60/0x90
__x64_sys_ioctl+0x16/0x20
do_syscall_64+0x49/0x130
entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reproducer:
  mkfs.btrfs -fq /dev/sdb && mount /dev/sdb /btrfs
  btrfs subvolume create /btrfs/sv1
  btrfs subvolume snapshot -r /btrfs/sv1 /btrfs/ss1
  umount /btrfs && mount -o ro /dev/sdb /btrfs
  btrfs send /btrfs/ss1 -f /tmp/f

Fix this by checking for the expected ORPHAN_CLEANUP_DONE only if the
filesystem is in writable state.

Reported-by: Christoph Anton Mitterer <calestyo@scientia.net>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
 fs/btrfs/send.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
index ae2db5eb1549..e3acec8aa8de 100644
--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -7085,9 +7085,11 @@ long btrfs_ioctl_send(struct file *mnt_file, struct btrfs_ioctl_send_args *arg)
 
 	/*
 	 * This is done when we lookup the root, it should already be complete
-	 * by the time we get here.
+	 * by the time we get here, unless the filesystem is readonly where the
+	 * orphan_cleanup_state is never started.
 	 */
-	WARN_ON(send_root->orphan_cleanup_state != ORPHAN_CLEANUP_DONE);
+	if (!sb_rdonly(file_inode(mnt_file)->i_sb))
+		WARN_ON(send_root->orphan_cleanup_state != ORPHAN_CLEANUP_DONE);
 
 	/*
 	 * Userspace tools do the checks and warn the user if it's
-- 
2.23.0