Btrfs: avoid races between super writeout and device list updates

On multi-device filesystems, btrfs writes supers to all of the devices
before considering a sync complete.  There wasn't any additional
locking between super writeout and the device list management code
because device management was done inside a transaction and
super writeout only happened  with no transation writers running.

With the btrfs fsync log and other async transaction updates, this
has been racey for some time.  This adds a mutex to protect
the device list.  The existing volume mutex could not be reused due to
transaction lock ordering requirements.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

3 files changed, 45 insertions, 2 deletions

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 6c54c210dfd0..b7ddc77fa568 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2111,7 +2111,7 @@ static int write_dev_supers(struct btrfs_device *device,
 
 int write_all_supers(struct btrfs_root *root, int max_mirrors)
 {
-        struct list_head *head = &root->fs_info->fs_devices->devices;
+        struct list_head *head;
         struct btrfs_device *dev;
         struct btrfs_super_block *sb;
         struct btrfs_dev_item *dev_item;
@@ -2126,6 +2126,9 @@ int write_all_supers(struct btrfs_root *root, int max_mirrors)
 
         sb = &root->fs_info->super_for_commit;
         dev_item = &sb->dev_item;
+
+        mutex_lock(&root->fs_info->fs_devices->device_list_mutex);
+        head = &root->fs_info->fs_devices->devices;
         list_for_each_entry(dev, head, dev_list) {
                 if (!dev->bdev) {
                         total_errors++;
@@ -2169,6 +2172,7 @@ int write_all_supers(struct btrfs_root *root, int max_mirrors)
                 if (ret)
                         total_errors++;
         }
+        mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
         if (total_errors > max_errors) {
                 printk(KERN_ERR "btrfs: %d errors while writing supers\n",
                        total_errors);
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 3f4a5932eac9..3ab80e9cd767 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -377,6 +377,7 @@ static noinline int device_list_add(const char *path,
                 memcpy(fs_devices->fsid, disk_super->fsid, BTRFS_FSID_SIZE);
                 fs_devices->latest_devid = devid;
                 fs_devices->latest_trans = found_transid;
+                mutex_init(&fs_devices->device_list_mutex);
                 device = NULL;
         } else {
                 device = __find_device(&fs_devices->devices, devid,
@@ -403,7 +404,11 @@ static noinline int device_list_add(const char *path,
                         return -ENOMEM;
                 }
                 INIT_LIST_HEAD(&device->dev_alloc_list);
+
+                mutex_lock(&fs_devices->device_list_mutex);
                 list_add(&device->dev_list, &fs_devices->devices);
+                mutex_unlock(&fs_devices->device_list_mutex);
+
                 device->fs_devices = fs_devices;
                 fs_devices->num_devices++;
         }
@@ -429,10 +434,12 @@ static struct btrfs_fs_devices *clone_fs_devices(struct btrfs_fs_devices *orig)
         INIT_LIST_HEAD(&fs_devices->devices);
         INIT_LIST_HEAD(&fs_devices->alloc_list);
         INIT_LIST_HEAD(&fs_devices->list);
+        mutex_init(&fs_devices->device_list_mutex);
         fs_devices->latest_devid = orig->latest_devid;
         fs_devices->latest_trans = orig->latest_trans;
         memcpy(fs_devices->fsid, orig->fsid, sizeof(fs_devices->fsid));
 
+        mutex_lock(&orig->device_list_mutex);
         list_for_each_entry(orig_dev, &orig->devices, dev_list) {
                 device = kzalloc(sizeof(*device), GFP_NOFS);
                 if (!device)
@@ -454,8 +461,10 @@ static struct btrfs_fs_devices *clone_fs_devices(struct btrfs_fs_devices *orig)
                 device->fs_devices = fs_devices;
                 fs_devices->num_devices++;
         }
+        mutex_unlock(&orig->device_list_mutex);
         return fs_devices;
 error:
+        mutex_unlock(&orig->device_list_mutex);
         free_fs_devices(fs_devices);
         return ERR_PTR(-ENOMEM);
 }
@@ -466,6 +475,7 @@ int btrfs_close_extra_devices(struct btrfs_fs_devices *fs_devices)
 
         mutex_lock(&uuid_mutex);
 again:
+        mutex_lock(&fs_devices->device_list_mutex);
         list_for_each_entry_safe(device, next, &fs_devices->devices, dev_list) {
                 if (device->in_fs_metadata)
                         continue;
@@ -485,6 +495,7 @@ again:
                 kfree(device->name);
                 kfree(device);
         }
+        mutex_unlock(&fs_devices->device_list_mutex);
 
         if (fs_devices->seed) {
                 fs_devices = fs_devices->seed;
@@ -1135,12 +1146,14 @@ int btrfs_rm_device(struct btrfs_root *root, char *device_path)
 
                 device = NULL;
                 devices = &root->fs_info->fs_devices->devices;
+                mutex_lock(&root->fs_info->fs_devices->device_list_mutex);
                 list_for_each_entry(tmp, devices, dev_list) {
                         if (tmp->in_fs_metadata && !tmp->bdev) {
                                 device = tmp;
                                 break;
                         }
                 }
+                mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
                 bdev = NULL;
                 bh = NULL;
                 disk_super = NULL;
@@ -1195,7 +1208,16 @@ int btrfs_rm_device(struct btrfs_root *root, char *device_path)
                 goto error_brelse;
 
         device->in_fs_metadata = 0;
+
+        /*
+         * the device list mutex makes sure that we don't change
+         * the device list while someone else is writing out all
+         * the device supers.
+         */
+        mutex_lock(&root->fs_info->fs_devices->device_list_mutex);
         list_del_init(&device->dev_list);
+        mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
+
         device->fs_devices->num_devices--;
 
         next_device = list_entry(root->fs_info->fs_devices->devices.next,
@@ -1289,6 +1311,7 @@ static int btrfs_prepare_sprout(struct btrfs_trans_handle *trans,
         seed_devices->opened = 1;
         INIT_LIST_HEAD(&seed_devices->devices);
         INIT_LIST_HEAD(&seed_devices->alloc_list);
+        mutex_init(&seed_devices->device_list_mutex);
         list_splice_init(&fs_devices->devices, &seed_devices->devices);
         list_splice_init(&fs_devices->alloc_list, &seed_devices->alloc_list);
         list_for_each_entry(device, &seed_devices->devices, dev_list) {
@@ -1414,6 +1437,10 @@ int btrfs_init_new_device(struct btrfs_root *root, char *device_path)
         mutex_lock(&root->fs_info->volume_mutex);
 
         devices = &root->fs_info->fs_devices->devices;
+        /*
+         * we have the volume lock, so we don't need the extra
+         * device list mutex while reading the list here.
+         */
         list_for_each_entry(device, devices, dev_list) {
                 if (device->bdev == bdev) {
                         ret = -EEXIST;
@@ -1468,6 +1495,12 @@ int btrfs_init_new_device(struct btrfs_root *root, char *device_path)
         }
 
         device->fs_devices = root->fs_info->fs_devices;
+
+        /*
+         * we don't want write_supers to jump in here with our device
+         * half setup
+         */
+        mutex_lock(&root->fs_info->fs_devices->device_list_mutex);
         list_add(&device->dev_list, &root->fs_info->fs_devices->devices);
         list_add(&device->dev_alloc_list,
                  &root->fs_info->fs_devices->alloc_list);
@@ -1486,6 +1519,7 @@ int btrfs_init_new_device(struct btrfs_root *root, char *device_path)
         total_bytes = btrfs_super_num_devices(&root->fs_info->super_copy);
         btrfs_set_super_num_devices(&root->fs_info->super_copy,
                                     total_bytes + 1);
+        mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
 
         if (seeding_dev) {
                 ret = init_first_rw_device(trans, root, device);
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 3c1f7310421e..5139a833f721 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -96,7 +96,12 @@ struct btrfs_fs_devices {
         u64 rw_devices;
         u64 total_rw_bytes;
         struct block_device *latest_bdev;
-        /* all of the devices in the FS */
+
+        /* all of the devices in the FS, protected by a mutex
+         * so we can safely walk it to write out the supers without
+         * worrying about add/remove by the multi-device code
+         */
+        struct mutex device_list_mutex;
         struct list_head devices;
 
         /* devices not currently being allocated */