btrfs: add and use helpers for reading and writing log_transid

Currently the log_transid field of a root is always modified while holding
the root's log_mutex locked. Most readers of a root's log_transid are also
holding the root's log_mutex locked, however there is one exception which
is btrfs_set_inode_last_trans() where we don't take the lock to avoid
blocking several operations if log syncing is happening in parallel.

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 log_transid field of a root 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>

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 3ebb5229660a3bbc08ac1c9b5613089f1682ede0..c8f1d2d7c46cd11858c0dbb12f3fdc5bbd423a08 100644 (file)
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -191,6 +191,14 @@ struct btrfs_root {
        atomic_t log_commit[2];
        /* Used only for log trees of subvolumes, not for the log root tree */
        atomic_t log_batch;
+       /*
+        * Protected by the 'log_mutex' lock but can be read without holding
+        * that lock to avoid unnecessary lock contention, in which case it
+        * should be read using btrfs_get_root_log_transid() except if it's a
+        * log tree in which case it can be directly accessed. Updates to this
+        * field should always use btrfs_set_root_log_transid(), except for log
+        * trees where the field can be updated directly.
+        */
        int log_transid;
        /* No matter the commit succeeds or not*/
        int log_transid_committed;
@@ -332,6 +340,16 @@ static inline u64 btrfs_root_id(const struct btrfs_root *root)
        return root->root_key.objectid;
 }
 
+static inline int btrfs_get_root_log_transid(const struct btrfs_root *root)
+{
+       return READ_ONCE(root->log_transid);
+}
+
+static inline void btrfs_set_root_log_transid(struct btrfs_root *root, int log_transid)
+{
+       WRITE_ONCE(root->log_transid, log_transid);
+}
+
 static inline int btrfs_get_root_last_log_commit(const struct btrfs_root *root)
 {
        return READ_ONCE(root->last_log_commit);

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index fe18c54cec1007e17a6e00da2ad5d1f99df4984c..c84d32951b2670bb8465abcefc1d3e32b41886bf 100644 (file)
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -675,7 +675,7 @@ static void __setup_root(struct btrfs_root *root, struct btrfs_fs_info *fs_info,
        refcount_set(&root->refs, 1);
        atomic_set(&root->snapshot_force_cow, 0);
        atomic_set(&root->nr_swapfiles, 0);
-       root->log_transid = 0;
+       btrfs_set_root_log_transid(root, 0);
        root->log_transid_committed = -1;
        btrfs_set_root_last_log_commit(root, 0);
        root->anon_dev = 0;
@@ -1004,7 +1004,7 @@ int btrfs_add_log_tree(struct btrfs_trans_handle *trans,
 
        WARN_ON(root->log_root);
        root->log_root = log_root;
-       root->log_transid = 0;
+       btrfs_set_root_log_transid(root, 0);
        root->log_transid_committed = -1;
        btrfs_set_root_last_log_commit(root, 0);
        return 0;

diff --git a/fs/btrfs/transaction.h b/fs/btrfs/transaction.h
index 365c0654890eaa7d5b30c615836429a6b2772881..18c4f6e83b78839d3a2f65306eef097f0f28a944 100644 (file)
--- a/fs/btrfs/transaction.h
+++ b/fs/btrfs/transaction.h
@@ -175,7 +175,7 @@ static inline void btrfs_set_inode_last_trans(struct btrfs_trans_handle *trans,
 {
        spin_lock(&inode->lock);
        inode->last_trans = trans->transaction->transid;
-       inode->last_sub_trans = inode->root->log_transid;
+       inode->last_sub_trans = btrfs_get_root_log_transid(inode->root);
        inode->last_log_commit = inode->last_sub_trans - 1;
        spin_unlock(&inode->lock);
 }

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 28a61a7dd371919ac98cb863f664ac317745aa65..958bb23d3d992014c3d45f1c885cac1911772681 100644 (file)
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -2958,7 +2958,7 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
        btrfs_set_root_node(&log->root_item, log->node);
        memcpy(&new_root_item, &log->root_item, sizeof(new_root_item));
 
-       root->log_transid++;
+       btrfs_set_root_log_transid(root, root->log_transid + 1);
        log->log_transid = root->log_transid;
        root->log_start_pid = 0;
        /*