btrfs: add and use helpers for reading and writing last_trans_committed

Currently the last_trans_committed field of struct btrfs_fs_info is
modified and read without any locking or other protection. For example
early in the fsync path, skip_inode_logging() is called which reads
fs_info->last_trans_committed, but at the same time we can have a
transaction commit completing and updating that field.

In the case of an fsync this is harmless and any data race should be
rare and at most cause an unnecessary logging of an inode.

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 last_trans_committed field of struct btrfs_fs_info using
READ_ONCE() and WRITE_ONCE(), and use these helpers everywhere.

[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/file.c b/fs/btrfs/file.c
index 723f0c70452e..92e6f224bff9 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1760,7 +1760,7 @@ static inline bool skip_inode_logging(const struct btrfs_log_ctx *ctx)
 	 * and for a fast fsync we don't wait for that, we only wait for the
 	 * writeback to complete.
 	 */
-	if (inode->last_trans <= fs_info->last_trans_committed &&
+	if (inode->last_trans <= btrfs_get_last_trans_committed(fs_info) &&
 	    (test_bit(BTRFS_INODE_NEEDS_FULL_SYNC, &inode->runtime_flags) ||
 	     list_empty(&ctx->ordered_extents)))
 		return true;