fs: rcu-walk aware d_revalidate method

--------------------------- dentry_operations --------------------------

prototypes:

	int (*d_revalidate)(struct dentry *, int);

	int (*d_revalidate)(struct dentry *, struct nameidata *);

	int (*d_hash)(const struct dentry *, const struct inode *,

			struct qstr *);

	int (*d_compare)(const struct dentry *, const struct inode *,

	char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);

locking rules:

		rename_lock	->d_lock	may block

d_revalidate:	no		no		yes

d_hash		no		no		no

d_compare:	yes		no		no

d_delete:	no		yes		no

d_release:	no		no		yes

d_iput:		no		no		yes

d_dname:	no		no		no

		rename_lock	->d_lock	may block	rcu-walk

d_revalidate:	no		no		yes (ref-walk)	maybe

d_hash		no		no		no		maybe

d_compare:	yes		no		no		maybe

d_delete:	no		yes		no		no

d_release:	no		no		yes		no

d_iput:		no		no		yes		no

d_dname:	no		no		no		no

--------------------------- inode_operations --------------------------- 

prototypes:

The cases where rcu-walk cannot continue are:

* NULL dentry (ie. any uncached path element)

* parent with d_inode->i_op->permission or ACLs

* dentries with d_revalidate

* Following links

In future patches, permission checks and d_revalidate become rcu-walk aware. It

may be possible eventually to make following links rcu-walk aware.

In future patches, permission checks become rcu-walk aware. It may be possible

eventually to make following links rcu-walk aware.

Uncached path elements will always require dropping to ref-walk mode, at the

very least because i_mutex needs to be grabbed, and objects allocated.

  INIT_LIST_HEAD(&inode->i_dentry);

must be done in the RCU callback.

--

[recommended]

	vfs now tries to do path walking in "rcu-walk mode", which avoids

atomic operations and scalability hazards on dentries and inodes (see

Documentation/filesystems/path-walk.txt). d_hash and d_compare changes (above)

are examples of the changes required to support this. For more complex

filesystem callbacks, the vfs drops out of rcu-walk mode before the fs call, so

no changes are required to the filesystem. However, this is costly and loses

the benefits of rcu-walk mode. We will begin to add filesystem callbacks that

are rcu-walk aware, shown below. Filesystems should take advantage of this

where possible.

--

[mandatory]

	d_revalidate is a callback that is made on every path element (if

the filesystem provides it), which requires dropping out of rcu-walk mode. This

may now be called in rcu-walk mode (nd->flags & LOOKUP_RCU). -ECHILD should be

returned if the filesystem cannot handle rcu-walk. See

Documentation/filesystems/vfs.txt for more details.

	dcache. Most filesystems leave this as NULL, because all their

	dentries in the dcache are valid

	d_revalidate may be called in rcu-walk mode (nd->flags & LOOKUP_RCU).

	If in rcu-walk mode, the filesystem must revalidate the dentry without

	blocking or storing to the dentry, d_parent and d_inode should not be

	used without care (because they can go NULL), instead nd->inode should

	be used.

	If a situation is encountered that rcu-walk cannot handle, return

	-ECHILD and it will be called again in ref-walk mode.

  d_hash: called when the VFS adds a dentry to the hash table. The first

	dentry passed to d_hash is the parent directory that the name is

	to be hashed into. The inode is the dentry's inode.

	struct autofs_dir_ent *ent;

	int res;

	if (nd->flags & LOOKUP_RCU)

		return -ECHILD;

	lock_kernel();

	dir = dentry->d_parent->d_inode;

	sbi = autofs_sbi(dir->i_sb);