Btrfs: using rcu lock in the reader side of devices list

fs_devices->devices is only updated on remove and add device paths, so we can
use rcu to protect it in the reader side

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 59 insertions, 26 deletions

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 0b5ca273726..e7844f8a347 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -363,7 +363,7 @@ static noinline int device_list_add(const char *path,
                 INIT_LIST_HEAD(&device->dev_alloc_list);
 
                 mutex_lock(&fs_devices->device_list_mutex);
-                list_add(&device->dev_list, &fs_devices->devices);
+                list_add_rcu(&device->dev_list, &fs_devices->devices);
                 mutex_unlock(&fs_devices->device_list_mutex);
 
                 device->fs_devices = fs_devices;
@@ -471,6 +471,29 @@ again:
         return 0;
 }
 
+static void __free_device(struct work_struct *work)
+{
+        struct btrfs_device *device;
+
+        device = container_of(work, struct btrfs_device, rcu_work);
+
+        if (device->bdev)
+                blkdev_put(device->bdev, device->mode);
+
+        kfree(device->name);
+        kfree(device);
+}
+
+static void free_device(struct rcu_head *head)
+{
+        struct btrfs_device *device;
+
+        device = container_of(head, struct btrfs_device, rcu);
+
+        INIT_WORK(&device->rcu_work, __free_device);
+        schedule_work(&device->rcu_work);
+}
+
 static int __btrfs_close_devices(struct btrfs_fs_devices *fs_devices)
 {
         struct btrfs_device *device;
@@ -480,18 +503,27 @@ static int __btrfs_close_devices(struct btrfs_fs_devices *fs_devices)
 
         mutex_lock(&fs_devices->device_list_mutex);
         list_for_each_entry(device, &fs_devices->devices, dev_list) {
-                if (device->bdev) {
-                        blkdev_put(device->bdev, device->mode);
+                struct btrfs_device *new_device;
+
+                if (device->bdev)
                         fs_devices->open_devices--;
-                }
+
                 if (device->writeable) {
                         list_del_init(&device->dev_alloc_list);
                         fs_devices->rw_devices--;
                 }
 
-                device->bdev = NULL;
-                device->writeable = 0;
-                device->in_fs_metadata = 0;
+                new_device = kmalloc(sizeof(*new_device), GFP_NOFS);
+                BUG_ON(!new_device);
+                memcpy(new_device, device, sizeof(*new_device));
+                new_device->name = kstrdup(device->name, GFP_NOFS);
+                BUG_ON(!new_device->name);
+                new_device->bdev = NULL;
+                new_device->writeable = 0;
+                new_device->in_fs_metadata = 0;
+                list_replace_rcu(&device->dev_list, &new_device->dev_list);
+
+                call_rcu(&device->rcu, free_device);
         }
         mutex_unlock(&fs_devices->device_list_mutex);
 
@@ -1204,11 +1236,13 @@ int btrfs_rm_device(struct btrfs_root *root, char *device_path)
         struct block_device *bdev;
         struct buffer_head *bh = NULL;
         struct btrfs_super_block *disk_super;
+        struct btrfs_fs_devices *cur_devices;
         u64 all_avail;
         u64 devid;
         u64 num_devices;
         u8 *dev_uuid;
         int ret = 0;
+        bool clear_super = false;
 
         mutex_lock(&uuid_mutex);
         mutex_lock(&root->fs_info->volume_mutex);
@@ -1294,6 +1328,7 @@ int btrfs_rm_device(struct btrfs_root *root, char *device_path)
                 list_del_init(&device->dev_alloc_list);
                 unlock_chunks(root);
                 root->fs_info->fs_devices->rw_devices--;
+                clear_super = true;
         }
 
         ret = btrfs_shrink_device(device, 0);
@@ -1304,16 +1339,15 @@ int btrfs_rm_device(struct btrfs_root *root, char *device_path)
         if (ret)
                 goto error_undo;
 
-        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.
          */
+
+        cur_devices = device->fs_devices;
         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);
+        list_del_rcu(&device->dev_list);
 
         device->fs_devices->num_devices--;
 
@@ -1327,36 +1361,36 @@ int btrfs_rm_device(struct btrfs_root *root, char *device_path)
         if (device->bdev == root->fs_info->fs_devices->latest_bdev)
                 root->fs_info->fs_devices->latest_bdev = next_device->bdev;
 
-        if (device->bdev) {
-                blkdev_put(device->bdev, device->mode);
-                device->bdev = NULL;
+        if (device->bdev)
                 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);
 
-        if (device->fs_devices->open_devices == 0) {
+        if (cur_devices->open_devices == 0) {
                 struct btrfs_fs_devices *fs_devices;
                 fs_devices = root->fs_info->fs_devices;
                 while (fs_devices) {
-                        if (fs_devices->seed == device->fs_devices)
+                        if (fs_devices->seed == cur_devices)
                                 break;
                         fs_devices = fs_devices->seed;
                 }
-                fs_devices->seed = device->fs_devices->seed;
-                device->fs_devices->seed = NULL;
+                fs_devices->seed = cur_devices->seed;
+                cur_devices->seed = NULL;
                 lock_chunks(root);
-                __btrfs_close_devices(device->fs_devices);
+                __btrfs_close_devices(cur_devices);
                 unlock_chunks(root);
-                free_fs_devices(device->fs_devices);
+                free_fs_devices(cur_devices);
         }
 
         /*
          * at this point, the device is zero sized.  We want to
          * remove it from the devices list and zero out the old super
          */
-        if (device->writeable) {
+        if (clear_super) {
                 /* make sure this device isn't detected as part of
                  * the FS anymore
                  */
@@ -1365,8 +1399,6 @@ int btrfs_rm_device(struct btrfs_root *root, char *device_path)
                 sync_dirty_buffer(bh);
         }
 
-        kfree(device->name);
-        kfree(device);
         ret = 0;
 
 error_brelse:
@@ -1425,7 +1457,8 @@ static int btrfs_prepare_sprout(struct btrfs_trans_handle *trans,
         mutex_init(&seed_devices->device_list_mutex);
 
         mutex_lock(&root->fs_info->fs_devices->device_list_mutex);
-        list_splice_init(&fs_devices->devices, &seed_devices->devices);
+        list_splice_init_rcu(&fs_devices->devices, &seed_devices->devices,
+                              synchronize_rcu);
         mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
 
         list_splice_init(&fs_devices->alloc_list, &seed_devices->alloc_list);
@@ -1624,7 +1657,7 @@ int btrfs_init_new_device(struct btrfs_root *root, char *device_path)
          * 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_rcu(&device->dev_list, &root->fs_info->fs_devices->devices);
         list_add(&device->dev_alloc_list,
                  &root->fs_info->fs_devices->alloc_list);
         root->fs_info->fs_devices->num_devices++;