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Subject[PATCH 4.18 151/197] btrfs: fix mount and ioctl device scan ioctl race
4.18-stable review patch.  If anyone has any objections, please let me know.


From: David Sterba <>

[ Upstream commit 81ffd56b5745355b70d54ca4e1bdd0d64a66ff9f ]

Technically this extends the critical section covered by uuid_mutex to:

- parse early mount options -- here we can call device scan on paths
that can be passed as 'device=/dev/...'

- scan the device passed to mount

- open the devices related to the fs_devices -- this increases

The race can happen when mount calls one of the scans and there's
another one called eg. by mkfs or 'btrfs dev scan':

Mount Scan
----- ----
scan_one_device (dev1, fsid1)
scan_one_device (dev2, fsid1)
add the device
free stale devices
fsid1 fs_devices::opened == 0
find fsid1:dev1
free fsid1:dev1
if it's the last one,
free fs_devices of fsid1

open_devices (dev1, fsid1)
dev1 not found

When fixed, the uuid mutex will make sure that mount will increase
fs_devices::opened and this will not be touched by the racing scan

Reviewed-by: Anand Jain <>
Signed-off-by: David Sterba <>

Signed-off-by: Sasha Levin <>
Signed-off-by: Greg Kroah-Hartman <>
fs/btrfs/super.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)

--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1557,19 +1557,19 @@ static struct dentry *btrfs_mount_root(s

error = btrfs_parse_early_options(data, mode, fs_type, &fs_devices);
- mutex_unlock(&uuid_mutex);
- if (error)
+ if (error) {
+ mutex_unlock(&uuid_mutex);
goto error_fs_info;
+ }

- mutex_lock(&uuid_mutex);
error = btrfs_scan_one_device(device_name, mode, fs_type, &fs_devices);
- mutex_unlock(&uuid_mutex);
- if (error)
+ if (error) {
+ mutex_unlock(&uuid_mutex);
goto error_fs_info;
+ }

fs_info->fs_devices = fs_devices;

- mutex_lock(&uuid_mutex);
error = btrfs_open_devices(fs_devices, mode, fs_type);
if (error)

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