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authorFilipe David Borba Manana <fdmanana@gmail.com>2013-08-09 16:41:36 +0200
committerChris Mason <chris.mason@fusionio.com>2013-09-01 14:16:24 +0200
commitd73068018419c5999f594a52998621947dc1f7d0 (patch)
treed5de1cca9d49b6af6ddfc21425d65393bb55c97e /fs/btrfs/volumes.c
parentBtrfs: remove ourselves from the cluster list under lock (diff)
downloadlinux-d73068018419c5999f594a52998621947dc1f7d0.tar.xz
linux-d73068018419c5999f594a52998621947dc1f7d0.zip
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>
Diffstat (limited to '')
-rw-r--r--fs/btrfs/volumes.c8
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;