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Btrfs: fix race between removing a dev and writing sbs

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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>
This commit is contained in:
Filipe David Borba Manana 2013-08-09 15:41:36 +01:00 committed by Chris Mason
parent b8d0c69b94
commit d730680184
2 changed files with 7 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
fs/btrfs/disk-io.c
View File

@ -3365,7 +3365,6 @@ static int write_all_supers(struct btrfs_root *root, int max_mirrors)
int total_errors = 0;
u64 flags;
max_errors = btrfs_super_num_devices(root->fs_info->super_copy) - 1;
do_barriers = !btrfs_test_opt(root, NOBARRIER);
backup_super_roots(root->fs_info);
@ -3374,6 +3373,7 @@ static int write_all_supers(struct btrfs_root *root, int max_mirrors)
mutex_lock(&root->fs_info->fs_devices->device_list_mutex);
head = &root->fs_info->fs_devices->devices;
max_errors = btrfs_super_num_devices(root->fs_info->super_copy) - 1;
if (do_barriers) {
ret = barrier_all_devices(root->fs_info);

8
fs/btrfs/volumes.c
View File

@ -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;