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btrfs: add and use helpers for reading and writing last_log_commit

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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>
This commit is contained in:
Filipe Manana 2023-10-04 11:38:48 +01:00 committed by David Sterba
parent a5b8a5f9f8
commit f985078796
4 changed files with 20 additions and 6 deletions
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2
fs/btrfs/btrfs_inode.h
View File

@ -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;

16
fs/btrfs/ctree.h
View File

@ -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.

4
fs/btrfs/disk-io.c
View File

@ -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;
}

4
fs/btrfs/tree-log.c
View File

@ -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);