vfs: kill FS_REVAL_DOT by adding a d_weak_revalidate dentry op

The following set of operations on a NFS client and server will cause

    server# mkdir a
    client# cd a
    server# mv a a.bak
    client# sleep 30  # (or whatever the dir attrcache timeout is)
    client# stat .
    stat: cannot stat `.': Stale NFS file handle

Obviously, we should not be getting an ESTALE error back there since the
inode still exists on the server. The problem is that the lookup code
will call d_revalidate on the dentry that "." refers to, because NFS has
FS_REVAL_DOT set.

nfs_lookup_revalidate will see that the parent directory has changed and
will try to reverify the dentry by redoing a LOOKUP. That of course
fails, so the lookup code returns ESTALE.

The problem here is that d_revalidate is really a bad fit for this case.
What we really want to know at this point is whether the inode is still
good or not, but we don't really care what name it goes by or whether
the dcache is still valid.

Add a new d_op->d_weak_revalidate operation and have complete_walk call
that instead of d_revalidate. The intent there is to allow for a
"weaker" d_revalidate that just checks to see whether the inode is still
good. This is also gives us an opportunity to kill off the FS_REVAL_DOT
special casing.

[AV: changed method name, added note in porting, fixed confusion re
having it possibly called from RCU mode (it won't be)]

Cc: NeilBrown <neilb@suse.de>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

3 files changed, 28 insertions, 2 deletions

diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index f48e0c6b4c42..0706d32a61e6 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -10,6 +10,7 @@ be able to use diff(1).
 --------------------------- dentry_operations --------------------------
 prototypes:
         int (*d_revalidate)(struct dentry *, unsigned int);
+        int (*d_weak_revalidate)(struct dentry *, unsigned int);
         int (*d_hash)(const struct dentry *, const struct inode *,
                         struct qstr *);
         int (*d_compare)(const struct dentry *, const struct inode *,
@@ -25,6 +26,7 @@ prototypes:
 locking rules:
                 rename_lock     ->d_lock        may block       rcu-walk
 d_revalidate:   no              no              yes (ref-walk)  maybe
+d_weak_revalidate:no            no              yes             no
 d_hash          no              no              no              maybe
 d_compare:      yes             no              no              maybe
 d_delete:       no              yes             no              no
diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting
index 0472c31c163b..4db22f6491e0 100644
--- a/Documentation/filesystems/porting
+++ b/Documentation/filesystems/porting
@@ -441,3 +441,7 @@ d_make_root() drops the reference to inode if dentry allocation fails.
 two, it gets "is it an O_EXCL or equivalent?" boolean argument.  Note that
 local filesystems can ignore tha argument - they are guaranteed that the
 object doesn't exist.  It's remote/distributed ones that might care...
+--
+[mandatory]
+        FS_REVAL_DOT is gone; if you used to have it, add ->d_weak_revalidate()
+in your dentry operations instead.
diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index e3869098163e..bc4b06b3160a 100644
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -900,6 +900,7 @@ defined:
 
 struct dentry_operations {
         int (*d_revalidate)(struct dentry *, unsigned int);
+        int (*d_weak_revalidate)(struct dentry *, unsigned int);
         int (*d_hash)(const struct dentry *, const struct inode *,
                         struct qstr *);
         int (*d_compare)(const struct dentry *, const struct inode *,
@@ -915,8 +916,13 @@ struct dentry_operations {
 
   d_revalidate: called when the VFS needs to revalidate a dentry. This
         is called whenever a name look-up finds a dentry in the
-        dcache. Most filesystems leave this as NULL, because all their
-        dentries in the dcache are valid
+        dcache. Most local filesystems leave this as NULL, because all their
+        dentries in the dcache are valid. Network filesystems are different
+        since things can change on the server without the client necessarily
+        being aware of it.
+
+        This function should return a positive value if the dentry is still
+        valid, and zero or a negative error code if it isn't.
 
         d_revalidate may be called in rcu-walk mode (flags & LOOKUP_RCU).
         If in rcu-walk mode, the filesystem must revalidate the dentry without
@@ -927,6 +933,20 @@ struct dentry_operations {
         If a situation is encountered that rcu-walk cannot handle, return
         -ECHILD and it will be called again in ref-walk mode.
 
+ d_weak_revalidate: called when the VFS needs to revalidate a "jumped" dentry.
+        This is called when a path-walk ends at dentry that was not acquired by
+        doing a lookup in the parent directory. This includes "/", "." and "..",
+        as well as procfs-style symlinks and mountpoint traversal.
+
+        In this case, we are less concerned with whether the dentry is still
+        fully correct, but rather that the inode is still valid. As with
+        d_revalidate, most local filesystems will set this to NULL since their
+        dcache entries are always valid.
+
+        This function has the same return code semantics as d_revalidate.
+
+        d_weak_revalidate is only called after leaving rcu-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.