|VFS_RENAME(9)||The Linux VFS||VFS_RENAME(9)|
vfs_rename - rename a filesystem object
int vfs_rename(struct inode * old_dir, struct dentry * old_dentry, struct inode * new_dir, struct dentry * new_dentry, struct inode ** delegated_inode, unsigned int flags);
The caller must hold multiple mutexes--see lock_rename).
If vfs_rename discovers a delegation in need of breaking at either the source or destination, it will return -EWOULDBLOCK and return a reference to the inode in delegated_inode. The caller should then break the delegation and retry. Because breaking a delegation may take a long time, the caller should drop all locks before doing so.
Alternatively, a caller may pass NULL for delegated_inode. This may be appropriate for callers that expect the underlying filesystem not to be NFS exported.
The worst of all namespace operations - renaming directory. “Perverted” doesn't even start to describe it. Somebody in UCB had a heck of a trip... Problems: a) we can get into loop creation. b) race potential - two innocent renames can create a loop together. That's where 4.4 screws up. Current fix: serialization on sb->s_vfs_rename_mutex. We might be more accurate, but that's another story. c) we have to lock _four_ objects - parents and victim (if it exists), and source (if it is not a directory). And that - after we got ->i_mutex on parents (until then we don't know whether the target exists). Solution: try to be smart with locking order for inodes. We rely on the fact that tree topology may change only under ->s_vfs_rename_mutex _and_ that parent of the object we move will be locked. Thus we can rank directories by the tree (ancestors first) and rank all non-directories after them. That works since everybody except rename does “lock parent, lookup, lock child” and rename is under ->s_vfs_rename_mutex. HOWEVER, it relies on the assumption that any object with ->lookup has no more than 1 dentry. If “hybrid” objects will ever appear, we'd better make sure that there's no link(2) for them. d) conversion from fhandle to dentry may come in the wrong moment - when we are removing the target. Solution: we will have to grab ->i_mutex in the fhandle_to_dentry code. [FIXME - current nfsfh.c relies on ->i_mutex on parents, which works but leads to some truly excessive locking].
|January 2017||Kernel Hackers Manual 4.8.|