Btrfs: race free update of commit root for ro snapshots
This is a better solution for the problem addressed in the following
commit:
Btrfs: update commit root on snapshot creation after orphan cleanup
(3821f348889e506efbd268cc8149e0ebfa47c4e5)
The previous solution wasn't the best because of 2 reasons:
1) It added another full transaction commit, which is more expensive
than just swapping the commit root with the root;
2) If a reboot happened after the first transaction commit (the one
that creates the snapshot) and before the second transaction commit,
then we would end up with the same problem if a send using that
snapshot was requested before the first transaction commit after
the reboot.
This change addresses those 2 issues. The second issue is addressed by
switching the commit root in the dentry lookup VFS callback, which is
also called by the snapshot/subvol creation ioctl and performs orphan
cleanup if needed. Like the vfs, the ioctl locks the parent inode too,
preventing race issues between a dentry lookup and snapshot creation.
Cc: Alex Lyakas <alex.btrfs@zadarastorage.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index a3c6e76..6dd6e50 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -5181,6 +5181,42 @@
iput(inode);
inode = ERR_PTR(ret);
}
+ /*
+ * If orphan cleanup did remove any orphans, it means the tree
+ * was modified and therefore the commit root is not the same as
+ * the current root anymore. This is a problem, because send
+ * uses the commit root and therefore can see inode items that
+ * don't exist in the current root anymore, and for example make
+ * calls to btrfs_iget, which will do tree lookups based on the
+ * current root and not on the commit root. Those lookups will
+ * fail, returning a -ESTALE error, and making send fail with
+ * that error. So make sure a send does not see any orphans we
+ * have just removed, and that it will see the same inodes
+ * regardless of whether a transaction commit happened before
+ * it started (meaning that the commit root will be the same as
+ * the current root) or not.
+ */
+ if (sub_root->node != sub_root->commit_root) {
+ u64 sub_flags = btrfs_root_flags(&sub_root->root_item);
+
+ if (sub_flags & BTRFS_ROOT_SUBVOL_RDONLY) {
+ struct extent_buffer *eb;
+
+ /*
+ * Assert we can't have races between dentry
+ * lookup called through the snapshot creation
+ * ioctl and the VFS.
+ */
+ ASSERT(mutex_is_locked(&dir->i_mutex));
+
+ down_write(&root->fs_info->commit_root_sem);
+ eb = sub_root->commit_root;
+ sub_root->commit_root =
+ btrfs_root_node(sub_root);
+ up_write(&root->fs_info->commit_root_sem);
+ free_extent_buffer(eb);
+ }
+ }
}
return inode;