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>

diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h
index 81bf514d988fee625aac592d63c1840b0486d6db..d32ef248828eee9d59f2b9efd2f77e630081cb7d 100644 (file)
--- a/fs/btrfs/btrfs_inode.h
+++ b/fs/btrfs/btrfs_inode.h
@@ -390,7 +390,7 @@ static inline bool btrfs_inode_in_log(struct btrfs_inode *inode, u64 generation)
        spin_lock(&inode->lock);
        if (inode->logged_trans == generation &&
            inode->last_sub_trans <= inode->last_log_commit &&
-           inode->last_sub_trans <= inode->root->last_log_commit)
+           inode->last_sub_trans <= btrfs_get_root_last_log_commit(inode->root))
                ret = true;
        spin_unlock(&inode->lock);
        return ret;

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 208a1888ca076bd9e406f6883e7594b822e45bcd..3ebb5229660a3bbc08ac1c9b5613089f1682ede0 100644 (file)
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -194,7 +194,11 @@ struct btrfs_root {
        int log_transid;
        /* No matter the commit succeeds or not*/
        int log_transid_committed;
-       /* Just be updated when the commit succeeds. */
+       /*
+        * Just be updated when the commit succeeds. Use
+        * btrfs_get_root_last_log_commit() and btrfs_set_root_last_log_commit()
+        * to access this field.
+        */
        int last_log_commit;
        pid_t log_start_pid;
 
@@ -328,6 +332,16 @@ static inline u64 btrfs_root_id(const struct btrfs_root *root)
        return root->root_key.objectid;
 }
 
+static inline int btrfs_get_root_last_log_commit(const struct btrfs_root *root)
+{
+       return READ_ONCE(root->last_log_commit);
+}
+
+static inline void btrfs_set_root_last_log_commit(struct btrfs_root *root, int commit_id)
+{
+       WRITE_ONCE(root->last_log_commit, commit_id);
+}
+
 /*
  * Structure that conveys information about an extent that is going to replace
  * all the extents in a file range.

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index ff3802986b3efd0821a4cbb8bd2e51c8990cacd3..fe18c54cec1007e17a6e00da2ad5d1f99df4984c 100644 (file)
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -677,7 +677,7 @@ static void __setup_root(struct btrfs_root *root, struct btrfs_fs_info *fs_info,
        atomic_set(&root->nr_swapfiles, 0);
        root->log_transid = 0;
        root->log_transid_committed = -1;
-       root->last_log_commit = 0;
+       btrfs_set_root_last_log_commit(root, 0);
        root->anon_dev = 0;
        if (!dummy) {
                extent_io_tree_init(fs_info, &root->dirty_log_pages,
@@ -1006,7 +1006,7 @@ int btrfs_add_log_tree(struct btrfs_trans_handle *trans,
        root->log_root = log_root;
        root->log_transid = 0;
        root->log_transid_committed = -1;
-       root->last_log_commit = 0;
+       btrfs_set_root_last_log_commit(root, 0);
        return 0;
 }
 

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 2c4685316c431bdfa48c574cac8b550ff1276061..28a61a7dd371919ac98cb863f664ac317745aa65 100644 (file)
--- 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);