fs: change d_compare for rcu-walk

Change d_compare so it may be called from lock-free RCU lookups. This
does put significant restrictions on what may be done from the callback,
however there don't seem to have been any problems with in-tree fses.
If some strange use case pops up that _really_ cannot cope with the
rcu-walk rules, we can just add new rcu-unaware callbacks, which would
cause name lookup to drop out of rcu-walk mode.

For in-tree filesystems, this is just a mechanical change.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>

1 files changed, 23 insertions, 3 deletions

diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index 95c0a93f056c..250681b8c7cc 100644
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -848,7 +848,9 @@ defined:
 struct dentry_operations {
         int (*d_revalidate)(struct dentry *, struct nameidata *);
         int (*d_hash)(struct dentry *, struct qstr *);
-        int (*d_compare)(struct dentry *, struct qstr *, struct qstr *);
+        int (*d_compare)(const struct dentry *, const struct inode *,
+                        const struct dentry *, const struct inode *,
+                        unsigned int, const char *, const struct qstr *);
         int (*d_delete)(const struct dentry *);
         void (*d_release)(struct dentry *);
         void (*d_iput)(struct dentry *, struct inode *);
@@ -860,9 +862,27 @@ struct dentry_operations {
         dcache. Most filesystems leave this as NULL, because all their
         dentries in the dcache are valid
 
-  d_hash: called when the VFS adds a dentry to the hash table
+  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.
 
-  d_compare: called when a dentry should be compared with another
+  d_compare: called to compare a dentry name with a given name. The first
+        dentry is the parent of the dentry to be compared, the second is
+        the parent's inode, then the dentry and inode (may be NULL) of the
+        child dentry. len and name string are properties of the dentry to be
+        compared. qstr is the name to compare it with.
+
+        Must be constant and idempotent, and should not take locks if
+        possible, and should not or store into the dentry or inodes.
+        Should not dereference pointers outside the dentry or inodes without
+        lots of care (eg.  d_parent, d_inode, d_name should not be used).
+
+        However, our vfsmount is pinned, and RCU held, so the dentries and
+        inodes won't disappear, neither will our sb or filesystem module.
+        ->i_sb and ->d_sb may be used.
+
+        It is a tricky calling convention because it needs to be called under
+        "rcu-walk", ie. without any locks or references on things.
 
   d_delete: called when the last reference to a dentry is dropped and the
         dcache is deciding whether or not to cache it. Return 1 to delete