vfs: call d_op->d_prune() before unhashing dentry

The d_prune dentry operation is used to notify filesystem when VFS
about to prune a hashed dentry from the dcache. There are three
code paths that prune dentries: shrink_dcache_for_umount_subtree(),
prune_dcache_sb() and d_prune_aliases(). For the d_prune_aliases()
case, VFS unhashes the dentry first, then call the d_prune dentry
operation. This confuses ceph_d_prune() (ceph uses the d_prune
dentry operation to maintain a flag indicating whether the complete
contents of a directory are in the dcache, pruning unhashed dentry
does not affect dir's completeness)

This patch fixes the issue by calling the d_prune dentry operation
in d_prune_aliases(), before unhashing the dentry. Also make VFS
only call the d_prune dentry operation for hashed dentry, to avoid
calling the d_prune dentry operation twice when dentry is pruned
by d_prune_aliases().
Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -472,7 +472,7 @@ static inline struct dentry *dentry_kill(struct dentry *dentry, int ref)
 	 * inform the fs via d_prune that this dentry is about to be
 	 * unhashed and destroyed.
 	 */
-	if (dentry->d_flags & DCACHE_OP_PRUNE)
+	if ((dentry->d_flags & DCACHE_OP_PRUNE) && !d_unhashed(dentry))
 		dentry->d_op->d_prune(dentry);
 
 	dentry_lru_del(dentry);
@@ -719,6 +719,14 @@ void d_prune_aliases(struct inode *inode)
 	hlist_for_each_entry(dentry, &inode->i_dentry, d_alias) {
 		spin_lock(&dentry->d_lock);
 		if (!dentry->d_count) {
+			/*
+			 * inform the fs via d_prune that this dentry
+			 * is about to be unhashed and destroyed.
+			 */
+			if ((dentry->d_flags & DCACHE_OP_PRUNE) &&
+			    !d_unhashed(dentry))
+				dentry->d_op->d_prune(dentry);
+
 			__dget_dlock(dentry);
 			__d_drop(dentry);
 			spin_unlock(&dentry->d_lock);
@@ -907,7 +915,8 @@ static void shrink_dcache_for_umount_subtree(struct dentry *dentry)
 			 * inform the fs that this dentry is about to be
 			 * unhashed and destroyed.
 			 */
-			if (dentry->d_flags & DCACHE_OP_PRUNE)
+			if ((dentry->d_flags & DCACHE_OP_PRUNE) &&
+			    !d_unhashed(dentry))
 				dentry->d_op->d_prune(dentry);
 
 			dentry_lru_del(dentry);