Re: races in ll_splice_alias()

[Al Viro <viro@ZenIV.linux.org.uk>]

On Tue, Mar 08, 2016 at 08:44:24PM +0000, Drokin, Oleg wrote:

> The links are hardlinks, right? (because otherwise they would not be
> parallel due to parent dir i_mutex held).

Yes.

> Hm, The race is a "safe" one, since if the alias have appeared, it does not break
> anything other than using up some RAM, I think?
> In fact what's to stop one appearing after we released the locking leading to the
> same situation?

Kinda-sorta.  It's safe unless a rename on server gets you see the same
directory in two places.  d_splice_alias() would've coped with that, this
code won't.

> > If so, how about adding d_hash_exact_alias(dentry) that would try to find
> > and hash an exact unhashed alias, so that this thing would
> > 	* call d_hash_exact_alias(dentry), if found - just return it
> > 	* ll_d_init(dentry)
> > 	* return d_splice_alias() ?: dentry
> > Do you see any problems with that?  Parent directory is locked, so no new
> > unhashed exact aliases should have a chance to appear and d_splice_alias()
> > would take care of everything else (correctness and detached ones).
> 
> This sounds fine on the surface. I think I remember there were some other
> complications with d_splice_alias.
> Andreas, do ou remember?

FWIW, a patch in my queue kills d_add_unique(), replacing it with
d_exact_alias() and d_splice_alias(); it could be used to implement
ll_splice_alias() as well (with code duplication gone *and* capable
of dealing with directories moving around), except for the odd
rules re inode refcount on error; it would've been easier if ll_splice_alias()
would always either inserted inode reference into a new dentry or dropped it.
I'm still trying to trace what does iput() in case of error in your current
code; I understand the one in do_statahead_enter(), but what does it in
ll_lookup_it_finish()?

Anyway, d_add_unique() removal (and switching its only caller to replacement)
follows:

replace d_add_unique() with saner primitive

new primitive: d_exact_alias(dentry, inode).  If there is an unhashed
dentry with the same name/parent and given inode, rehash, grab and
return it.  Otherwise, return NULL.  The only caller of d_add_unique()
switched to d_exact_alias() + d_splice_alias().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/dcache.c b/fs/dcache.c
index 2398f9f9..4d20bf5 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1782,81 +1782,6 @@ void d_instantiate(struct dentry *entry, struct inode * inode)
 EXPORT_SYMBOL(d_instantiate);
 
 /**
- * d_instantiate_unique - instantiate a non-aliased dentry
- * @entry: dentry to instantiate
- * @inode: inode to attach to this dentry
- *
- * Fill in inode information in the entry. On success, it returns NULL.
- * If an unhashed alias of "entry" already exists, then we return the
- * aliased dentry instead and drop one reference to inode.
- *
- * Note that in order to avoid conflicts with rename() etc, the caller
- * had better be holding the parent directory semaphore.
- *
- * This also assumes that the inode count has been incremented
- * (or otherwise set) by the caller to indicate that it is now
- * in use by the dcache.
- */
-static struct dentry *__d_instantiate_unique(struct dentry *entry,
-					     struct inode *inode)
-{
-	struct dentry *alias;
-	int len = entry->d_name.len;
-	const char *name = entry->d_name.name;
-	unsigned int hash = entry->d_name.hash;
-
-	if (!inode) {
-		__d_instantiate(entry, NULL);
-		return NULL;
-	}
-
-	hlist_for_each_entry(alias, &inode->i_dentry, d_u.d_alias) {
-		/*
-		 * Don't need alias->d_lock here, because aliases with
-		 * d_parent == entry->d_parent are not subject to name or
-		 * parent changes, because the parent inode i_mutex is held.
-		 */
-		if (alias->d_name.hash != hash)
-			continue;
-		if (alias->d_parent != entry->d_parent)
-			continue;
-		if (alias->d_name.len != len)
-			continue;
-		if (dentry_cmp(alias, name, len))
-			continue;
-		__dget(alias);
-		return alias;
-	}
-
-	__d_instantiate(entry, inode);
-	return NULL;
-}
-
-struct dentry *d_instantiate_unique(struct dentry *entry, struct inode *inode)
-{
-	struct dentry *result;
-
-	BUG_ON(!hlist_unhashed(&entry->d_u.d_alias));
-
-	if (inode)
-		spin_lock(&inode->i_lock);
-	result = __d_instantiate_unique(entry, inode);
-	if (inode)
-		spin_unlock(&inode->i_lock);
-
-	if (!result) {
-		security_d_instantiate(entry, inode);
-		return NULL;
-	}
-
-	BUG_ON(!d_unhashed(result));
-	iput(inode);
-	return result;
-}
-
-EXPORT_SYMBOL(d_instantiate_unique);
-
-/**
  * d_instantiate_no_diralias - instantiate a non-aliased dentry
  * @entry: dentry to complete
  * @inode: inode to attach to this dentry
@@ -2437,6 +2362,56 @@ void d_rehash(struct dentry * entry)
 EXPORT_SYMBOL(d_rehash);
 
 /**
+ * d_exact_alias - find and hash an exact unhashed alias
+ * @entry: dentry to add
+ * @inode: The inode to go with this dentry
+ *
+ * If an unhashed dentry with the same name/parent and desired
+ * inode already exists, hash and return it.  Otherwise, return
+ * NULL.
+ *
+ * Parent directory should be locked.
+ */
+struct dentry *d_exact_alias(struct dentry *entry, struct inode *inode)
+{
+	struct dentry *alias;
+	int len = entry->d_name.len;
+	const char *name = entry->d_name.name;
+	unsigned int hash = entry->d_name.hash;
+
+	spin_lock(&inode->i_lock);
+	hlist_for_each_entry(alias, &inode->i_dentry, d_u.d_alias) {
+		/*
+		 * Don't need alias->d_lock here, because aliases with
+		 * d_parent == entry->d_parent are not subject to name or
+		 * parent changes, because the parent inode i_mutex is held.
+		 */
+		if (alias->d_name.hash != hash)
+			continue;
+		if (alias->d_parent != entry->d_parent)
+			continue;
+		if (alias->d_name.len != len)
+			continue;
+		if (dentry_cmp(alias, name, len))
+			continue;
+		spin_lock(&alias->d_lock);
+		if (!d_unhashed(alias)) {
+			spin_unlock(&alias->d_lock);
+			alias = NULL;
+		} else {
+			__dget_dlock(alias);
+			_d_rehash(alias);
+			spin_unlock(&alias->d_lock);
+		}
+		spin_unlock(&inode->i_lock);
+		return alias;
+	}
+	spin_unlock(&inode->i_lock);
+	return NULL;
+}
+EXPORT_SYMBOL(d_exact_alias);
+
+/**
  * dentry_update_name_case - update case insensitive dentry with a new name
  * @dentry: dentry to be updated
  * @name: new name
diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
index 4bfc33a..9a5d67f 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -2461,14 +2461,16 @@ static int _nfs4_open_and_get_state(struct nfs4_opendata *opendata,
 
 	dentry = opendata->dentry;
 	if (d_really_is_negative(dentry)) {
-		/* FIXME: Is this d_drop() ever needed? */
+		struct dentry *alias;
 		d_drop(dentry);
-		dentry = d_add_unique(dentry, igrab(state->inode));
-		if (dentry == NULL) {
-			dentry = opendata->dentry;
-		} else if (dentry != ctx->dentry) {
+		alias = d_exact_alias(dentry, state->inode);
+		if (!alias)
+			alias = d_splice_alias(igrab(state->inode), dentry);
+		/* d_splice_alias() can't fail here - it's a non-directory */
+		if (alias) {
+			dentry = dget(alias);
 			dput(ctx->dentry);
-			ctx->dentry = dget(dentry);
+			ctx->dentry = dentry;
 		}
 		nfs_set_verifier(dentry,
 				nfs_save_change_attribute(d_inode(opendata->dir)));
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index c4b5f4b..bda4ec5 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -246,6 +246,7 @@ extern struct dentry * d_alloc(struct dentry *, const struct qstr *);
 extern struct dentry * d_alloc_pseudo(struct super_block *, const struct qstr *);
 extern struct dentry * d_splice_alias(struct inode *, struct dentry *);
 extern struct dentry * d_add_ci(struct dentry *, struct inode *, struct qstr *);
+extern struct dentry * d_exact_alias(struct dentry *, struct inode *);
 extern struct dentry *d_find_any_alias(struct inode *inode);
 extern struct dentry * d_obtain_alias(struct inode *);
 extern struct dentry * d_obtain_root(struct inode *);
@@ -288,23 +289,6 @@ static inline void d_add(struct dentry *entry, struct inode *inode)
 	d_rehash(entry);
 }
 
-/**
- * d_add_unique - add dentry to hash queues without aliasing
- * @entry: dentry to add
- * @inode: The inode to attach to this dentry
- *
- * This adds the entry to the hash queues and initializes @inode.
- * The entry was actually filled in earlier during d_alloc().
- */
-static inline struct dentry *d_add_unique(struct dentry *entry, struct inode *inode)
-{
-	struct dentry *res;
-
-	res = d_instantiate_unique(entry, inode);
-	d_rehash(res != NULL ? res : entry);
-	return res;
-}
-
 extern void dentry_update_name_case(struct dentry *, struct qstr *);
 
 /* used for rename() and baskets */