btrfs: add and use helpers for reading and writing last_log_commit

Currently, the last_log_commit of a root can be accessed concurrently
without any lock protection. Readers can be calling btrfs_inode_in_log()
early in a fsync call, which reads a root's last_log_commit, while a
writer can change the last_log_commit while a log tree if being synced,
at btrfs_sync_log(). Any 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.

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_log_commit field of a root 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, 2 insertions, 2 deletions

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 2c4685316c43..28a61a7dd371 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -3133,8 +3133,8 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
 	 * someone else already started it. We use <= and not < because the
 	 * first log transaction has an ID of 0.
 	 */
-	ASSERT(root->last_log_commit <= log_transid);
-	root->last_log_commit = log_transid;
+	ASSERT(btrfs_get_root_last_log_commit(root) <= log_transid);
+	btrfs_set_root_last_log_commit(root, log_transid);
 
 out_wake_log_root:
 	mutex_lock(&log_root_tree->log_mutex);