Btrfs: fix race between removing a dev and writing sbs

This change fixes an issue when removing a device and writing
all super blocks run simultaneously. Here's the steps necessary
for the issue to happen:

1) disk-io.c:write_all_supers() gets a number of N devices from the
   super_copy, so it will not panic if it fails to write super blocks
   for N - 1 devices;

2) Then it tries to acquire the device_list_mutex, but blocks because
   volumes.c:btrfs_rm_device() got it first;

3) btrfs_rm_device() removes the device from the list, then unlocks the
   mutex and after the unlock it updates the number of devices in
   super_copy to N - 1.

4) write_all_supers() finally acquires the mutex, iterates over all the
   devices in the list and gets N - 1 errors, that is, it failed to write
   super blocks to all the devices;

5) Because write_all_supers() thinks there are a total of N devices, it
   considers N - 1 errors to be ok, and therefore won't panic.

So this change just makes sure that write_all_supers() reads the number
of devices from super_copy after it acquires the device_list_mutex.
Conversely, it changes btrfs_rm_device() to update the number of devices
in super_copy before it releases the device list mutex.

The code path to add a new device (volumes.c:btrfs_init_new_device),
already has the right behaviour: it updates the number of devices in
super_copy while holding the device_list_mutex.

The only code path that doesn't lock the device list mutex
before updating the number of devices in the super copy is
disk-io.c:next_root_backup(), called by open_ctree() during
mount time where concurrency issues can't happen.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 34068b887f14..3f1c2c200691 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1620,7 +1620,11 @@ int btrfs_rm_device(struct btrfs_root *root, char *device_path)
         /*
          * the device list mutex makes sure that we don't change
          * the device list while someone else is writing out all
-         * the device supers.
+         * the device supers. Whoever is writing all supers, should
+         * lock the device list mutex before getting the number of
+         * devices in the super block (super_copy). Conversely,
+         * whoever updates the number of devices in the super block
+         * (super_copy) should hold the device list mutex.
          */
 
         cur_devices = device->fs_devices;
@@ -1644,10 +1648,10 @@ int btrfs_rm_device(struct btrfs_root *root, char *device_path)
                 device->fs_devices->open_devices--;
 
         call_rcu(&device->rcu, free_device);
-        mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
 
         num_devices = btrfs_super_num_devices(root->fs_info->super_copy) - 1;
         btrfs_set_super_num_devices(root->fs_info->super_copy, num_devices);
+        mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
 
         if (cur_devices->open_devices == 0) {
                 struct btrfs_fs_devices *fs_devices;