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btrfs: switch to RCU for device traversal in btrfs_ioctl_dev_info 

We don't need to use the mutex as we do not modify the devices nor the
list itself and just read some information:

does not change during device lifetime:
- devid
- uuid
- name (ie. the path)

may change in parallel to the ioctl call, but can lead only to reporting
inacurracy:
- bytes_used
- total_bytes

Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
David Sterba 2017-06-16 00:09:21 +02:00
parent 08ffcae8c9
commit c5593ca3c8
1 changed files with 2 additions and 5 deletions
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7
fs/btrfs/ioctl.c
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@ -2775,7 +2775,6 @@ static long btrfs_ioctl_dev_info(struct btrfs_fs_info *fs_info,
{
struct btrfs_ioctl_dev_info_args *di_args;
struct btrfs_device *dev;
struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
int ret = 0;
char *s_uuid = NULL;
@ -2786,7 +2785,7 @@ static long btrfs_ioctl_dev_info(struct btrfs_fs_info *fs_info,
if (!btrfs_is_empty_uuid(di_args->uuid))
s_uuid = di_args->uuid;
mutex_lock(&fs_devices->device_list_mutex);
rcu_read_lock();
dev = btrfs_find_device(fs_info, di_args->devid, s_uuid, NULL);
if (!dev) {
@ -2801,17 +2800,15 @@ static long btrfs_ioctl_dev_info(struct btrfs_fs_info *fs_info,
if (dev->name) {
struct rcu_string *name;
rcu_read_lock();
name = rcu_dereference(dev->name);
strncpy(di_args->path, name->str, sizeof(di_args->path));
rcu_read_unlock();
di_args->path[sizeof(di_args->path) - 1] = 0;
} else {
di_args->path[0] = '\0';
}
out:
mutex_unlock(&fs_devices->device_list_mutex);
rcu_read_unlock();
if (ret == 0 && copy_to_user(arg, di_args, sizeof(*di_args)))
ret = -EFAULT;