btrfs: add and use helpers for reading and writing fs_info->generation

Currently the generation field of struct btrfs_fs_info is always modified
while holding fs_info->trans_lock locked. Most readers will access this
field without taking that lock but while holding a transaction handle,
which is safe to do due to the transaction life cycle.

However there are other readers that are neither holding the lock nor
holding a transaction handle open:

1) When reading an inode from disk, at btrfs_read_locked_inode();

2) When reading the generation to expose it to sysfs, at
   btrfs_generation_show();

3) Early in the fsync path, at skip_inode_logging();

4) When creating a hole at btrfs_cont_expand(), during write paths,
   truncate and reflinking;

5) In the fs_info ioctl (btrfs_ioctl_fs_info());

6) While mounting the filesystem, in the open_ctree() path. In these
   cases it's safe to directly read fs_info->generation as no one
   can concurrently start a transaction and update fs_info->generation.

In case of the fsync path, races here should be harmless, and in the worst
case they may cause a fsync to log an inode when it's not really needed,
so nothing bad from a functional perspective. In the other cases it's not
so clear if functional problems may arise, though in case 1 rare things
like a load/store tearing [1] may cause the BTRFS_INODE_NEEDS_FULL_SYNC
flag not being set on an inode and therefore result in incorrect logging
later on in case a fsync call is made.

To avoid data race warnings from tools like KCSAN and other issues such
as load and store tearing (amongst others, see [1]), create helpers to
access the generation field of struct btrfs_fs_info using READ_ONCE() and
WRITE_ONCE(), and use these helpers where needed.

[1] https://lwn.net/Articles/793253/

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 848b7e6f6421..7ab21283fae8 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2822,7 +2822,7 @@ static long btrfs_ioctl_fs_info(struct btrfs_fs_info *fs_info,
 	}
 
 	if (flags_in & BTRFS_FS_INFO_FLAG_GENERATION) {
-		fi_args->generation = fs_info->generation;
+		fi_args->generation = btrfs_get_fs_generation(fs_info);
 		fi_args->flags |= BTRFS_FS_INFO_FLAG_GENERATION;
 	}