Merge tag 'pull-dcache' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs

Pull dcache updates from Al Viro:

 - d_alloc_parallel() API change (Neil's with my changes)

 - NORCU fixes

 - Reorganization and simplification of dentry eviction logic

 - Simplifying rcu_read_lock() scopes in fs/dcache.c

 - Secondary roots work - getting rid of NFS fake root dentries and
   dealing with remaining shrink_dcache_for_umount() and
   shrink_dentry_list() races

 - making cursors NORCU (surprisingly easy)

* tag 'pull-dcache' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs: (22 commits)
  make cursors NORCU
  nfs: get rid of fake root dentries
  wind ->s_roots via ->d_sib instead of ->d_hash
  shrink_dentry_tree(): unify the calls of shrink_dentry_list()
  shrinking rcu_read_lock() scope in d_alloc_parallel()
  d_walk(): shrink rcu_read_lock() scope
  document dentry_kill()
  adjust calling conventions of lock_for_kill(), fold __dentry_kill() into dentry_kill()
  Document rcu_read_lock() use in select_collect2()
  Shift rcu_read_{,un}lock() inside fast_dput()
  simplify safety for lock_for_kill() slowpath
  fold lock_for_kill() and __dentry_kill() into common helper
  fold lock_for_kill() into shrink_kill()
  shrink_dentry_list(): start with removing from shrink list
  d_prune_aliases(): make sure to skip NORCU aliases
  kill d_dispose_if_unused()
  make to_shrink_list() return whether it has moved dentry to list
  select_collect(): ignore dentries on shrink lists if they have positive refcounts
  find_acceptable_alias(): skip NORCU aliases with zero refcount
  fix a race between d_find_any_alias() and final dput() of NORCU dentries
  ...

Documentation/filesystems/porting.rst

@@ -1384,3 +1384,20 @@ for_each_alias(dentry, inode) instead of hlist_for_each_entry; better
 yet, see if any of the exported primitives could be used instead of
 the entire loop.  You still need to hold ->i_lock of the inode over
 either form of manual loop.
+
+---
+
+**mandatory**
+
+d_alloc_parallel() no longer requires a waitqueue_head.
+
+---
+
+**mandatory**
+
+d_dispose_if_unused() is gone; use __move_to_shrink_list() if you really
+need that functionality, but watch out for memory safety issues - just
+as with d_dispose_if_unused() these are not trivial; with this variant
+of API it's more explicit, since grabbing ->d_lock is caller-side, but
+d_dispose_if_unused() had all the same issues.  It's a low-level primitive;
+use only if you have no alternative.

fs/afs/dir_silly.c

@@ -248,13 +248,11 @@ int afs_silly_iput(struct dentry *dentry, struct inode *inode)
 	struct dentry *alias;
 	int ret;
 
-	DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
-
 	_enter("%p{%pd},%llx", dentry, dentry, vnode->fid.vnode);
 
 	down_read(&dvnode->rmdir_lock);
 
-	alias = d_alloc_parallel(dentry->d_parent, &dentry->d_name, &wq);
+	alias = d_alloc_parallel(dentry->d_parent, &dentry->d_name);
 	if (IS_ERR(alias)) {
 		up_read(&dvnode->rmdir_lock);
 		return 0;

fs/dcache.c

@@ -43,8 +43,8 @@
  *   - i_dentry, d_alias, d_inode of aliases
  * dcache_hash_bucket lock protects:
  *   - the dcache hash table
- * s_roots bl list spinlock protects:
+ * s_roots_lock protects:
- *   - the s_roots list (see __d_drop)
+ *   - the s_roots list (see __d_move()/dentry_unlist()/d_obtain_root())
  * dentry->d_sb->s_dentry_lru_lock protects:
  *   - the dcache lru lists and counters
  * d_lock protects:
@@ -462,14 +462,10 @@ static void dentry_unlink_inode(struct dentry * dentry)
 
 	raw_write_seqcount_begin(&dentry->d_seq);
 	__d_clear_type_and_inode(dentry);
-	hlist_del_init(&dentry->d_alias);
+	__hlist_del(&dentry->d_alias);
 	/*
 	 * dentry becomes negative, so the space occupied by ->d_alias
-	 * belongs to ->waiters now; we could use __hlist_del() instead
+	 * belongs to ->waiters now.
-	 * of hlist_del_init(), if not for the stunt pulled by nfs
-	 * dummy root dentries - positive dentry *not* included into
-	 * the alias list of its inode.  Open-coding hlist_del_init()
-	 * and removing zeroing would be too clumsy...
 	 */
 	dentry->waiters = NULL;
 	raw_write_seqcount_end(&dentry->d_seq);
@@ -568,16 +564,7 @@ static void d_lru_shrink_move(struct list_lru_one *lru, struct dentry *dentry,
 
 static void ___d_drop(struct dentry *dentry)
 {
-	struct hlist_bl_head *b;
+	struct hlist_bl_head *b = d_hash(dentry->d_name.hash);
-	/*
-	 * Hashed dentries are normally on the dentry hashtable,
-	 * with the exception of those newly allocated by
-	 * d_obtain_root, which are always IS_ROOT:
-	 */
-	if (unlikely(IS_ROOT(dentry)))
-		b = &dentry->d_sb->s_roots;
-	else
-		b = d_hash(dentry->d_name.hash);
 
 	hlist_bl_lock(b);
 	__hlist_bl_del(&dentry->d_hash);
@@ -636,12 +623,14 @@ struct completion_list {
  *  Use ->waiters for a single-linked list of struct completion_list of
  *  waiters.
  */
-static inline void d_add_waiter(struct dentry *dentry, struct completion_list *p)
+static inline bool d_add_waiter(struct dentry *dentry, struct completion_list *p)
 {
-	struct completion_list *v = dentry->waiters;
+	if (unlikely(dentry->d_flags & DCACHE_DENTRY_KILLED))
+		return false;
 	init_completion(&p->completion);
-	p->next = v;
+	p->next = dentry->waiters;
 	dentry->waiters = p;
+	return true;
 }
 
 static inline void d_complete_waiters(struct dentry *dentry)
@@ -658,6 +647,13 @@ static inline void d_complete_waiters(struct dentry *dentry)
 	}
 }
 
+static void unlink_secondary_root(struct dentry *dentry)
+{
+	spin_lock(&dentry->d_sb->s_roots_lock);
+	hlist_del_init(&dentry->d_sib);
+	spin_unlock(&dentry->d_sb->s_roots_lock);
+}
+
 static inline void dentry_unlist(struct dentry *dentry)
 {
 	struct dentry *next;
@@ -669,6 +665,10 @@ static inline void dentry_unlist(struct dentry *dentry)
 	d_complete_waiters(dentry);
 	if (unlikely(hlist_unhashed(&dentry->d_sib)))
 		return;
+	if (unlikely(IS_ROOT(dentry))) {
+		unlink_secondary_root(dentry); // secondary root goes away
+		return;
+	}
 	__hlist_del(&dentry->d_sib);
 	/*
 	 * Cursors can move around the list of children.  While we'd been
@@ -697,11 +697,113 @@ static inline void dentry_unlist(struct dentry *dentry)
 	}
 }
 
-static struct dentry *__dentry_kill(struct dentry *dentry)
+/*
+ * Prepare locking environment for killing a dentry.
+ * Called under dentry->d_lock.  To proceed with eviction of a positive dentry
+ * we need to get ->i_lock of the inode of that dentry as well.
+ * However, ->i_lock nests outside of ->d_lock, so if trylock fails we might
+ * have to drop and regain the latter.  Dentry state can change while its
+ * ->d_lock is not held - it might end up getting killed, becoming busy,
+ * negative, etc., so we need to be careful.
+ *
+ * For NORCU dentries memory safety relies upon having only one call of
+ * lock_for_kill() in the entire lifetime of dentry and dentry_free() being
+ * called only by the caller of lock_for_kill().  That this is NORCU-specific;
+ * the crucial part is that refcounts of NORCU dentries never grow once having
+ * dropped to zero.
+ *
+ * For normal dentries we can not assume that there won't be concurrent calls
+ * of dentry_free() - dentry might end up being evicted by another thread
+ * while we are dropping/retaking locks on the slow path.  Memory safety is
+ * provided by keeping the RCU read-side critical area contiguous with
+ * an explicit rcu_read_lock() scope bridging over the break in spinlock scopes.
+ *
+ * If dentry is busy (or busy dying, or already dead), unlock dentry
+ * and return false.  Otherwise, return true and have that dentry's
+ * inode (if any) locked in addition to dentry itself.
+ */
+static bool lock_for_kill(struct dentry *dentry)
+{
+	struct inode *inode = dentry->d_inode;
+
+	if (unlikely(dentry->d_lockref.count)) {
+		spin_unlock(&dentry->d_lock);
+		return false;
+	}
+
+	if (!inode || likely(spin_trylock(&inode->i_lock)))
+		return true;
+
+	// Too bad - we need to drop ->d_lock and take locks in correct order.
+	// To avoid breaking RCU read-side critical area when we drop ->d_lock,
+	// take an explicit rcu_read_lock() while we are switching locks.
+	rcu_read_lock();
+	do {
+		spin_unlock(&dentry->d_lock);
+		spin_lock(&inode->i_lock);
+		spin_lock(&dentry->d_lock);
+		// make sure we'd locked the right inode - ->d_inode might've
+		// changed while we were not holding ->d_lock
+		if (likely(inode == dentry->d_inode))
+			break;
+		spin_unlock(&inode->i_lock);
+		inode = dentry->d_inode;
+	} while (inode);
+	rcu_read_unlock();
+	if (likely(!dentry->d_lockref.count))
+		return true;
+	if (inode)
+		spin_unlock(&inode->i_lock);
+	spin_unlock(&dentry->d_lock);
+	return false;
+}
+
+/**
+ * dentry_kill - evict a dentry
+ * @dentry:	dentry to be evicted
+ *
+ * All dentry evictions are done by this function.  The reference we are
+ * passed does not contribute to the refcount; the caller had either
+ * already decremented the refcount or it had never held one in the
+ * first place.  @dentry->d_lock is held by the caller and dropped
+ * by dentry_kill(@dentry).
+ *
+ * We are guaranteed that nobody had called dentry_free(@dentry)
+ * prior to the beginning of RCU read-side critical area we are in.
+ *
+ * Caller must not access @dentry after the call.
+ *
+ * If eviction of @dentry drops the last reference to its parent,
+ * the reference to parent is returned to caller.  In that case
+ * it is guaranteed to satisfy the requirements for dentry_kill()
+ * argument - its ->d_lock is held and we are guaranteed that nobody
+ * had passed it to dentry_free() prior to acquisition of its ->d_lock.
+ * Otherwise %NULL is returned.
+ *
+ * If @dentry is idle and remains such after we assemble the full
+ * locking environment for eviction (see lock_for_kill() for details)
+ * we mark it doomed (->d_lockref.count < 0) and proceed to detaching
+ * it from any filesystem objects.  Otherwise we drop ->d_lock and
+ * return %NULL.
+ *
+ * Once @dentry is detached from the filesystem objects, we complete
+ * detaching it from dentry tree. The parent, if any, gets locked
+ * and its refcount is decremented; dentry is carefully removed from
+ * the tree (see dentry_unlist() for details) and marked killed
+ * (%DCACHE_DENTRY_KILLED set in ->d_flags).  At that point it's just
+ * an inert chunk of memory, accessible only via RCU references
+ * and possibly via a shrink list.  If it is not on any shrink lists,
+ * we call dentry_free(), which schedules actual freeing of memory.
+ * Othewise freeing is left to the owner of the shrink list in question.
+ */
+static struct dentry *dentry_kill(struct dentry *dentry)
 {
 	struct dentry *parent = NULL;
 	bool can_free = true;
 
+	if (unlikely(!lock_for_kill(dentry)))
+		return NULL;
+
 	/*
 	 * The dentry is now unrecoverably dead to the world.
 	 */
@@ -748,43 +850,6 @@ static struct dentry *__dentry_kill(struct dentry *dentry)
 	return parent;
 }
 
-/*
- * Lock a dentry for feeding it to __dentry_kill().
- * Called under rcu_read_lock() and dentry->d_lock; the former
- * guarantees that nothing we access will be freed under us.
- * Note that dentry is *not* protected from concurrent dentry_kill(),
- * d_delete(), etc.
- *
- * Return false if dentry is busy.  Otherwise, return true and have
- * that dentry's inode locked.
- */
-
-static bool lock_for_kill(struct dentry *dentry)
-{
-	struct inode *inode = dentry->d_inode;
-
-	if (unlikely(dentry->d_lockref.count))
-		return false;
-
-	if (!inode || likely(spin_trylock(&inode->i_lock)))
-		return true;
-
-	do {
-		spin_unlock(&dentry->d_lock);
-		spin_lock(&inode->i_lock);
-		spin_lock(&dentry->d_lock);
-		if (likely(inode == dentry->d_inode))
-			break;
-		spin_unlock(&inode->i_lock);
-		inode = dentry->d_inode;
-	} while (inode);
-	if (likely(!dentry->d_lockref.count))
-		return true;
-	if (inode)
-		spin_unlock(&inode->i_lock);
-	return false;
-}
-
 /*
  * Decide if dentry is worth retaining.  Usually this is called with dentry
  * locked; if not locked, we are more limited and might not be able to tell
@@ -860,17 +925,17 @@ EXPORT_SYMBOL(d_mark_dontcache);
  * If unsuccessful, we return false, having already taken the dentry lock.
  * In that case refcount is guaranteed to be zero and we have already
  * decided that it's not worth keeping around.
- *
- * The caller needs to hold the RCU read lock, so that the dentry is
- * guaranteed to stay around even if the refcount goes down to zero!
  */
 static inline bool fast_dput(struct dentry *dentry)
 {
 	int ret;
 
 	/*
-	 * try to decrement the lockref optimistically.
+	 * Try to decrement the lockref optimistically.
+	 * RCU read lock held so that dentry is guaranteed to stay around
+	 * even if the refcount goes down to zero.
 	 */
+	rcu_read_lock();
 	ret = lockref_put_return(&dentry->d_lockref);
 
 	/*
@@ -880,6 +945,7 @@ static inline bool fast_dput(struct dentry *dentry)
 	 */
 	if (unlikely(ret < 0)) {
 		spin_lock(&dentry->d_lock);
+		rcu_read_unlock();
 		if (WARN_ON_ONCE(dentry->d_lockref.count <= 0)) {
 			spin_unlock(&dentry->d_lock);
 			return true;
@@ -891,8 +957,10 @@ static inline bool fast_dput(struct dentry *dentry)
 	/*
 	 * If we weren't the last ref, we're done.
 	 */
-	if (ret)
+	if (ret) {
+		rcu_read_unlock();
 		return true;
+	}
 
 	/*
 	 * Can we decide that decrement of refcount is all we needed without
@@ -900,8 +968,10 @@ static inline bool fast_dput(struct dentry *dentry)
 	 * dentry looks like it ought to be retained and there's nothing else
 	 * to do.
 	 */
-	if (retain_dentry(dentry, false))
+	if (retain_dentry(dentry, false)) {
+		rcu_read_unlock();
 		return true;
+	}
 
 	/*
 	 * Either not worth retaining or we can't tell without the lock.
@@ -909,6 +979,7 @@ static inline bool fast_dput(struct dentry *dentry)
 	 * but we'll need to re-check the situation after getting the lock.
 	 */
 	spin_lock(&dentry->d_lock);
+	rcu_read_unlock();
 
 	/*
 	 * Did somebody else grab a reference to it in the meantime, and
@@ -926,21 +997,13 @@ locked:
 
 static void finish_dput(struct dentry *dentry)
 	__releases(dentry->d_lock)
-	__releases(RCU)
 {
-	while (lock_for_kill(dentry)) {
+	while ((dentry = dentry_kill(dentry)) != NULL) {
-		rcu_read_unlock();
-		dentry = __dentry_kill(dentry);
-		if (!dentry)
-			return;
 		if (retain_dentry(dentry, true)) {
 			spin_unlock(&dentry->d_lock);
 			return;
 		}
-		rcu_read_lock();
 	}
-	rcu_read_unlock();
-	spin_unlock(&dentry->d_lock);
 }
 
 /* 
@@ -974,11 +1037,8 @@ void dput(struct dentry *dentry)
 	if (!dentry)
 		return;
 	might_sleep();
-	rcu_read_lock();
+	if (likely(fast_dput(dentry)))
-	if (likely(fast_dput(dentry))) {
-		rcu_read_unlock();
 		return;
-	}
 	finish_dput(dentry);
 }
 EXPORT_SYMBOL(dput);
@@ -989,30 +1049,46 @@ void d_make_discardable(struct dentry *dentry)
 	WARN_ON(!(dentry->d_flags & DCACHE_PERSISTENT));
 	dentry->d_flags &= ~DCACHE_PERSISTENT;
 	dentry->d_lockref.count--;
-	rcu_read_lock();
 	finish_dput(dentry);
 }
 EXPORT_SYMBOL(d_make_discardable);
 
-static void to_shrink_list(struct dentry *dentry, struct list_head *list)
+/**
+ * __move_to_shrink_list - try to place a dentry into a shrink list
+ * @dentry:	dentry to try putting into shrink list
+ * @list:	the list to put @dentry into.
+ * Returns:	true @dentry had been placed into @list, false otherwise
+ *
+ * If @dentry is idle and not already include into a shrink list, move
+ * it into @list and return %true; otherwise do nothing and return %false.
+ *
+ * Caller must be holding @dentry->d_lock.  There must have been no calls of
+ * dentry_free(@dentry) prior to the beginning of the RCU read-side critical
+ * area in which __move_to_shrink_list(@dentry, @list) is called.
+ *
+ * @list should be thread-private and eventually emptied by passing it to
+ * shrink_dentry_list().
+ */
+
+bool __move_to_shrink_list(struct dentry *dentry, struct list_head *list)
 __must_hold(&dentry->d_lock)
 {
-	if (!(dentry->d_flags & DCACHE_SHRINK_LIST)) {
+	if (likely(!dentry->d_lockref.count &&
+	    !(dentry->d_flags & DCACHE_SHRINK_LIST))) {
 		if (dentry->d_flags & DCACHE_LRU_LIST)
 			d_lru_del(dentry);
 		d_shrink_add(dentry, list);
+		return true;
 	}
+	return false;
 }
+EXPORT_SYMBOL(__move_to_shrink_list);
 
 void dput_to_list(struct dentry *dentry, struct list_head *list)
 {
-	rcu_read_lock();
+	if (likely(fast_dput(dentry)))
-	if (likely(fast_dput(dentry))) {
-		rcu_read_unlock();
 		return;
-	}
+	__move_to_shrink_list(dentry, list);
-	rcu_read_unlock();
-	to_shrink_list(dentry, list);
 	spin_unlock(&dentry->d_lock);
 }
 
@@ -1058,7 +1134,10 @@ repeat:
 }
 EXPORT_SYMBOL(dget_parent);
 
-static struct dentry * __d_find_any_alias(struct inode *inode)
+/*
+ * inode is a directory, inode->i_lock is held by the caller
+ */
+static struct dentry * __d_find_dir_alias(struct inode *inode)
 {
 	struct dentry *alias;
 
@@ -1069,6 +1148,18 @@ static struct dentry * __d_find_any_alias(struct inode *inode)
 	return alias;
 }
 
+static struct dentry * __d_find_any_alias(struct inode *inode)
+{
+	struct dentry *alias;
+
+	if (hlist_empty(&inode->i_dentry))
+		return NULL;
+	for_each_alias(alias, inode)
+		if (dget_alias_ilocked(alias))
+			return alias;
+	return NULL;
+}
+
 /**
  * d_find_any_alias - find any alias for a given inode
  * @inode: inode to find an alias for
@@ -1092,7 +1183,7 @@ static struct dentry *__d_find_alias(struct inode *inode)
 	struct dentry *alias;
 
 	if (S_ISDIR(inode->i_mode))
-		return __d_find_any_alias(inode);
+		return __d_find_dir_alias(inode);
 
 	for_each_alias(alias, inode) {
 		spin_lock(&alias->d_lock);
@@ -1158,25 +1249,6 @@ struct dentry *d_find_alias_rcu(struct inode *inode)
 	return de;
 }
 
-/**
- * d_dispose_if_unused - move unreferenced dentries to shrink list
- * @dentry: dentry in question
- * @dispose: head of shrink list
- *
- * If dentry has no external references, move it to shrink list.
- *
- * NOTE!!! The caller is responsible for preventing eviction of the dentry by
- * holding dentry->d_inode->i_lock or equivalent.
- */
-void d_dispose_if_unused(struct dentry *dentry, struct list_head *dispose)
-{
-	spin_lock(&dentry->d_lock);
-	if (!dentry->d_lockref.count)
-		to_shrink_list(dentry, dispose);
-	spin_unlock(&dentry->d_lock);
-}
-EXPORT_SYMBOL(d_dispose_if_unused);
-
 /*
  *	Try to kill dentries associated with this inode.
  * WARNING: you must own a reference to inode.
@@ -1187,8 +1259,12 @@ void d_prune_aliases(struct inode *inode)
 	struct dentry *dentry;
 
 	spin_lock(&inode->i_lock);
-	for_each_alias(dentry, inode)
+	for_each_alias(dentry, inode) {
-		d_dispose_if_unused(dentry, &dispose);
+		spin_lock(&dentry->d_lock);
+		if (likely(!(dentry->d_flags & DCACHE_NORCU)))
+			__move_to_shrink_list(dentry, &dispose);
+		spin_unlock(&dentry->d_lock);
+	}
 	spin_unlock(&inode->i_lock);
 	shrink_dentry_list(&dispose);
 }
@@ -1196,14 +1272,8 @@ EXPORT_SYMBOL(d_prune_aliases);
 
 static inline void shrink_kill(struct dentry *victim)
 {
-	do {
+	while ((victim = dentry_kill(victim)) != NULL)
-		rcu_read_unlock();
+		;
-		victim = __dentry_kill(victim);
-		rcu_read_lock();
-	} while (victim && lock_for_kill(victim));
-	rcu_read_unlock();
-	if (victim)
-		spin_unlock(&victim->d_lock);
 }
 
 void shrink_dentry_list(struct list_head *list)
@@ -1213,18 +1283,12 @@ void shrink_dentry_list(struct list_head *list)
 
 		dentry = list_entry(list->prev, struct dentry, d_lru);
 		spin_lock(&dentry->d_lock);
-		rcu_read_lock();
+		d_shrink_del(dentry);
-		if (!lock_for_kill(dentry)) {
+		if (unlikely(dentry->d_flags & DCACHE_DENTRY_KILLED)) {
-			bool can_free;
-			rcu_read_unlock();
-			d_shrink_del(dentry);
-			can_free = dentry->d_flags & DCACHE_DENTRY_KILLED;
 			spin_unlock(&dentry->d_lock);
-			if (can_free)
+			dentry_free(dentry);
-				dentry_free(dentry);
 			continue;
 		}
-		d_shrink_del(dentry);
 		shrink_kill(dentry);
 	}
 }
@@ -1385,6 +1449,8 @@ again:
 	read_seqbegin_or_lock(&rename_lock, &seq);
 	this_parent = parent;
 	spin_lock(&this_parent->d_lock);
+	if (unlikely(this_parent->d_flags & DCACHE_DENTRY_CURSOR))
+		goto out_unlock;
 
 	ret = enter(data, this_parent);
 	switch (ret) {
@@ -1433,14 +1499,15 @@ resume:
 	/*
 	 * All done at this level ... ascend and resume the search.
 	 */
-	rcu_read_lock();
 ascend:
 	if (this_parent != parent) {
 		dentry = this_parent;
 		this_parent = dentry->d_parent;
 
+		rcu_read_lock();
 		spin_unlock(&dentry->d_lock);
 		spin_lock(&this_parent->d_lock);
+		rcu_read_unlock();
 
 		/* might go back up the wrong parent if we have had a rename. */
 		if (need_seqretry(&rename_lock, seq))
@@ -1448,7 +1515,6 @@ ascend:
 		/* go into the first sibling still alive */
 		hlist_for_each_entry_continue(dentry, d_sib) {
 			if (likely(!(dentry->d_flags & DCACHE_DENTRY_KILLED))) {
-				rcu_read_unlock();
 				goto resume;
 			}
 		}
@@ -1456,7 +1522,6 @@ ascend:
 	}
 	if (need_seqretry(&rename_lock, seq))
 		goto rename_retry;
-	rcu_read_unlock();
 
 out_unlock:
 	spin_unlock(&this_parent->d_lock);
@@ -1465,7 +1530,6 @@ out_unlock:
 
 rename_retry:
 	spin_unlock(&this_parent->d_lock);
-	rcu_read_unlock();
 	BUG_ON(seq & 1);
 	if (!retry)
 		return;
@@ -1580,12 +1644,8 @@ static enum d_walk_ret select_collect(void *_data, struct dentry *dentry)
 	if (data->start == dentry)
 		goto out;
 
-	if (dentry->d_flags & DCACHE_SHRINK_LIST) {
+	if (dentry->d_lockref.count <= 0) {
-		data->found++;
+		__move_to_shrink_list(dentry, &data->dispose);
-	} else if (!dentry->d_lockref.count) {
-		to_shrink_list(dentry, &data->dispose);
-		data->found++;
-	} else if (dentry->d_lockref.count < 0) {
 		data->found++;
 	}
 	/*
@@ -1616,17 +1676,21 @@ static enum d_walk_ret select_collect2(void *_data, struct dentry *dentry)
 	if (data->start == dentry)
 		goto out;
 
-	if (!dentry->d_lockref.count) {
+	if (dentry->d_lockref.count <= 0) {
-		if (dentry->d_flags & DCACHE_SHRINK_LIST) {
+		if (!__move_to_shrink_list(dentry, &data->dispose)) {
+			/*
+			 * We need an enter RCU read-side critical area that
+			 * would extend past the return from d_walk() and
+			 * we are in the scope of ->d_lock that will terminate
+			 * before that, so we use rcu_read_lock() to bridge
+			 * over to the scope of ->d_lock in d_walk() caller.
+			 * The scope of rcu_read_lock() spans from here to
+			 * paired rcu_read_unlock() in shrink_dcache_tree().
+			 */
 			rcu_read_lock();
 			data->victim = dentry;
 			return D_WALK_QUIT;
 		}
-		to_shrink_list(dentry, &data->dispose);
-	} else if (dentry->d_lockref.count < 0) {
-		rcu_read_lock();
-		data->victim = dentry;
-		return D_WALK_QUIT;
 	}
 	/*
 	 * We can return to the caller if we have found some (this
@@ -1649,7 +1713,9 @@ out:
 static void shrink_dcache_tree(struct dentry *parent, bool for_umount)
 {
 	for (;;) {
-		struct select_data data = {.start = parent};
+		struct completion_list wait;
+		bool need_wait = false;
+		struct select_data data = { .start = parent };
 
 		INIT_LIST_HEAD(&data.dispose);
 		d_walk(parent, &data,
@@ -1667,30 +1733,32 @@ static void shrink_dcache_tree(struct dentry *parent, bool for_umount)
 		d_walk(parent, &data, select_collect2);
 		if (data.victim) {
 			struct dentry *v = data.victim;
-
+			/*
+			 * select_collect2() has picked a dentry that was
+			 * either dying or on a shrink list and arranged
+			 * for it to be returned to us.  We are still in
+			 * the RCU read-side critical area started there
+			 * (rcu_read_lock() scope opened in select_collect2()),
+			 * so dentry couldn't have been freed yet, but its
+			 * state might've changed since we dropped ->d_lock
+			 * on the way out.  Switch over to ->d_lock scope
+			 * and recheck the dentry state.
+			 */
 			spin_lock(&v->d_lock);
-			if (v->d_lockref.count < 0 &&
+			rcu_read_unlock();
-			    !(v->d_flags & DCACHE_DENTRY_KILLED)) {
+
-				struct completion_list wait;
+			if (unlikely(v->d_lockref.count < 0)) {
-				// It's busy dying; have it notify us once
+				// It's doomed; if it isn't dead yet, notify us
-				// it becomes invisible to d_walk().
+				// once it becomes invisible to d_walk().
-				d_add_waiter(v, &wait);
+				need_wait = d_add_waiter(v, &wait);
 				spin_unlock(&v->d_lock);
-				rcu_read_unlock();
-				if (!list_empty(&data.dispose))
-					shrink_dentry_list(&data.dispose);
-				wait_for_completion(&wait.completion);
-				continue;
-			}
-			if (!lock_for_kill(v)) {
-				spin_unlock(&v->d_lock);
-				rcu_read_unlock();
 			} else {
 				shrink_kill(v);
 			}
 		}
-		if (!list_empty(&data.dispose))
+		shrink_dentry_list(&data.dispose);
-			shrink_dentry_list(&data.dispose);
+		if (unlikely(need_wait))
+			wait_for_completion(&wait.completion);
 	}
 }
 
@@ -1743,9 +1811,30 @@ void shrink_dcache_for_umount(struct super_block *sb)
 	sb->s_root = NULL;
 	do_one_tree(dentry);
 
-	while (!hlist_bl_empty(&sb->s_roots)) {
+	for (;;) {
-		dentry = dget(hlist_bl_entry(hlist_bl_first(&sb->s_roots), struct dentry, d_hash));
+		spin_lock(&sb->s_roots_lock);
-		do_one_tree(dentry);
+		dentry = hlist_entry_safe(sb->s_roots.first,
+					  struct dentry, d_sib);
+		if (!dentry) {
+			spin_unlock(&sb->s_roots_lock);
+			break;
+		}
+		rcu_read_lock();
+		spin_unlock(&sb->s_roots_lock);
+		spin_lock(&dentry->d_lock);
+		rcu_read_unlock();
+		if (unlikely(dentry->d_lockref.count < 0)) {
+			struct completion_list wait;
+			bool need_wait = d_add_waiter(dentry, &wait);
+
+			spin_unlock(&dentry->d_lock);
+			if (need_wait)
+				wait_for_completion(&wait.completion);
+		} else {
+			dget_dlock(dentry);
+			spin_unlock(&dentry->d_lock);
+			do_one_tree(dentry);
+		}
 	}
 }
 
@@ -1915,7 +2004,7 @@ struct dentry *d_alloc_cursor(struct dentry * parent)
 {
 	struct dentry *dentry = d_alloc_anon(parent->d_sb);
 	if (dentry) {
-		dentry->d_flags |= DCACHE_DENTRY_CURSOR;
+		dentry->d_flags |= DCACHE_DENTRY_CURSOR | DCACHE_NORCU;
 		dentry->d_parent = dget(parent);
 	}
 	return dentry;
@@ -2166,9 +2255,9 @@ static struct dentry *__d_obtain_alias(struct inode *inode, bool disconnected)
 		__d_set_inode_and_type(new, inode, add_flags);
 		hlist_add_head(&new->d_alias, &inode->i_dentry);
 		if (!disconnected) {
-			hlist_bl_lock(&sb->s_roots);
+			spin_lock(&sb->s_roots_lock);
-			hlist_bl_add_head(&new->d_hash, &sb->s_roots);
+			hlist_add_head(&new->d_sib, &sb->s_roots);
-			hlist_bl_unlock(&sb->s_roots);
+			spin_unlock(&sb->s_roots_lock);
 		}
 		spin_unlock(&new->d_lock);
 		spin_unlock(&inode->i_lock);
@@ -2260,8 +2349,7 @@ struct dentry *d_add_ci(struct dentry *dentry, struct inode *inode,
 		return found;
 	}
 	if (d_in_lookup(dentry)) {
-		found = d_alloc_parallel(dentry->d_parent, name,
+		found = d_alloc_parallel(dentry->d_parent, name);
-					dentry->d_wait);
 		if (IS_ERR(found) || !d_in_lookup(found)) {
 			iput(inode);
 			return found;
@@ -2648,32 +2736,24 @@ static inline unsigned start_dir_add(struct inode *dir)
 	}
 }
 
-static inline void end_dir_add(struct inode *dir, unsigned int n,
+static inline void end_dir_add(struct inode *dir, unsigned int n)
-			       wait_queue_head_t *d_wait)
 {
 	smp_store_release(&dir->i_dir_seq, n + 2);
 	preempt_enable_nested();
-	if (wq_has_sleeper(d_wait))
-		wake_up_all(d_wait);
 }
 
 static void d_wait_lookup(struct dentry *dentry)
 {
-	if (d_in_lookup(dentry)) {
+	if (likely(d_in_lookup(dentry))) {
-		DECLARE_WAITQUEUE(wait, current);
+		dentry->d_flags |= DCACHE_LOOKUP_WAITERS;
-		add_wait_queue(dentry->d_wait, &wait);
+		wait_var_event_spinlock(&dentry->d_flags,
-		do {
+					!d_in_lookup(dentry),
-			set_current_state(TASK_UNINTERRUPTIBLE);
+					&dentry->d_lock);
-			spin_unlock(&dentry->d_lock);
-			schedule();
-			spin_lock(&dentry->d_lock);
-		} while (d_in_lookup(dentry));
 	}
 }
 
 struct dentry *d_alloc_parallel(struct dentry *parent,
-				const struct qstr *name,
+				const struct qstr *name)
-				wait_queue_head_t *wq)
 {
 	unsigned int hash = name->hash;
 	struct hlist_bl_head *b = in_lookup_hash(parent, hash);
@@ -2694,38 +2774,33 @@ struct dentry *d_alloc_parallel(struct dentry *parent,
 	spin_unlock(&parent->d_lock);
 
 retry:
-	rcu_read_lock();
 	seq = smp_load_acquire(&parent->d_inode->i_dir_seq);
 	r_seq = read_seqbegin(&rename_lock);
+	rcu_read_lock();
 	dentry = __d_lookup_rcu(parent, name, &d_seq);
 	if (unlikely(dentry)) {
 		if (!lockref_get_not_dead(&dentry->d_lockref)) {
 			rcu_read_unlock();
 			goto retry;
 		}
+		rcu_read_unlock();
 		if (read_seqcount_retry(&dentry->d_seq, d_seq)) {
-			rcu_read_unlock();
 			dput(dentry);
 			goto retry;
 		}
-		rcu_read_unlock();
 		dput(new);
 		return dentry;
 	}
-	if (unlikely(read_seqretry(&rename_lock, r_seq))) {
+	rcu_read_unlock();
-		rcu_read_unlock();
+	if (unlikely(read_seqretry(&rename_lock, r_seq)))
 		goto retry;
-	}
 
-	if (unlikely(seq & 1)) {
+	if (unlikely(seq & 1))
-		rcu_read_unlock();
 		goto retry;
-	}
 
 	hlist_bl_lock(b);
 	if (unlikely(READ_ONCE(parent->d_inode->i_dir_seq) != seq)) {
 		hlist_bl_unlock(b);
-		rcu_read_unlock();
 		goto retry;
 	}
 	/*
@@ -2742,19 +2817,20 @@ retry:
 			continue;
 		if (!d_same_name(dentry, parent, name))
 			continue;
+		rcu_read_lock();
 		hlist_bl_unlock(b);
+		spin_lock(&dentry->d_lock);
+		rcu_read_unlock();
 		/* now we can try to grab a reference */
-		if (!lockref_get_not_dead(&dentry->d_lockref)) {
+		if (unlikely(dentry->d_lockref.count < 0)) {
-			rcu_read_unlock();
+			spin_unlock(&dentry->d_lock);
 			goto retry;
 		}
-
-		rcu_read_unlock();
 		/*
 		 * somebody is likely to be still doing lookup for it;
-		 * wait for them to finish
+		 * pin it and wait for them to finish
 		 */
-		spin_lock(&dentry->d_lock);
+		dget_dlock(dentry);
 		d_wait_lookup(dentry);
 		/*
 		 * it's not in-lookup anymore; in principle we should repeat
@@ -2775,8 +2851,6 @@ retry:
 		dput(new);
 		return dentry;
 	}
-	rcu_read_unlock();
-	new->d_wait = wq;
 	hlist_bl_add_head(&new->d_in_lookup_hash, b);
 	hlist_bl_unlock(b);
 	return new;
@@ -2788,13 +2862,26 @@ mismatch:
 EXPORT_SYMBOL(d_alloc_parallel);
 
 /*
- * - Unhash the dentry
+ * Move dentry from in-lookup state to busy-negative one.
- * - Retrieve and clear the waitqueue head in dentry
+ *
- * - Return the waitqueue head
+ * From now on d_in_lookup(dentry) will return false and dentry is gone from
+ * in-lookup hash.
+ *
+ * Anyone who had been waiting on it in d_alloc_parallel() is free to
+ * proceed after that.  Note that waking such waiters up is left to
+ * the callers; PREEMPT_RT kernels can't have that wakeup done while
+ * in write-side critical area for ->i_dir_seq, so it's done by calling
+ * __d_wake_in_lookup_waiters() once it's safe to do so.
+ *
+ * Both __d_lookup_unhash() and __d_wake_in_lookup_waiters() should
+ * be called within the same ->d_lock scope.  PAR_LOOKUP is cleared
+ * here, while LOOKUP_WAITERS (set by somebody finding dentry in
+ * the in-lookup hash and setting down to wait) is checked and cleared
+ * in __d_wake_in_lookup_waiters().  Both are gone by the end of
+ * ->d_lock scope.
  */
-static wait_queue_head_t *__d_lookup_unhash(struct dentry *dentry)
+static void __d_lookup_unhash(struct dentry *dentry)
 {
-	wait_queue_head_t *d_wait;
 	struct hlist_bl_head *b;
 
 	lockdep_assert_held(&dentry->d_lock);
@@ -2803,18 +2890,23 @@ static wait_queue_head_t *__d_lookup_unhash(struct dentry *dentry)
 	hlist_bl_lock(b);
 	dentry->d_flags &= ~DCACHE_PAR_LOOKUP;
 	__hlist_bl_del(&dentry->d_in_lookup_hash);
-	d_wait = dentry->d_wait;
-	dentry->d_wait = NULL;
 	hlist_bl_unlock(b);
 	dentry->waiters = NULL;
-	INIT_LIST_HEAD(&dentry->d_lru);
+}
-	return d_wait;
+
+static inline void __d_wake_in_lookup_waiters(struct dentry *dentry)
+{
+	if (dentry->d_flags & DCACHE_LOOKUP_WAITERS) {
+		wake_up_var_locked(&dentry->d_flags, &dentry->d_lock);
+		dentry->d_flags &= ~DCACHE_LOOKUP_WAITERS;
+	}
 }
 
 void __d_lookup_unhash_wake(struct dentry *dentry)
 {
 	spin_lock(&dentry->d_lock);
-	wake_up_all(__d_lookup_unhash(dentry));
+	__d_lookup_unhash(dentry);
+	__d_wake_in_lookup_waiters(dentry);
 	spin_unlock(&dentry->d_lock);
 }
 EXPORT_SYMBOL(__d_lookup_unhash_wake);
@@ -2824,14 +2916,13 @@ EXPORT_SYMBOL(__d_lookup_unhash_wake);
 static inline void __d_add(struct dentry *dentry, struct inode *inode,
 			   const struct dentry_operations *ops)
 {
-	wait_queue_head_t *d_wait;
 	struct inode *dir = NULL;
 	unsigned n;
 	spin_lock(&dentry->d_lock);
 	if (unlikely(d_in_lookup(dentry))) {
 		dir = dentry->d_parent->d_inode;
 		n = start_dir_add(dir);
-		d_wait = __d_lookup_unhash(dentry);
+		__d_lookup_unhash(dentry);
 	}
 	if (unlikely(ops))
 		d_set_d_op(dentry, ops);
@@ -2844,8 +2935,10 @@ static inline void __d_add(struct dentry *dentry, struct inode *inode,
 		fsnotify_update_flags(dentry);
 	}
 	__d_rehash(dentry);
-	if (dir)
+	if (dir) {
-		end_dir_add(dir, n, d_wait);
+		end_dir_add(dir, n);
+		__d_wake_in_lookup_waiters(dentry);
+	}
 	spin_unlock(&dentry->d_lock);
 	if (inode)
 		spin_unlock(&inode->i_lock);
@@ -2958,7 +3051,6 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
 		     bool exchange)
 {
 	struct dentry *old_parent, *p;
-	wait_queue_head_t *d_wait;
 	struct inode *dir = NULL;
 	unsigned n;
 
@@ -2989,7 +3081,7 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
 	if (unlikely(d_in_lookup(target))) {
 		dir = target->d_parent->d_inode;
 		n = start_dir_add(dir);
-		d_wait = __d_lookup_unhash(target);
+		__d_lookup_unhash(target);
 	}
 
 	write_seqcount_begin(&dentry->d_seq);
@@ -3028,9 +3120,10 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
 	write_seqcount_end(&target->d_seq);
 	write_seqcount_end(&dentry->d_seq);
 
-	if (dir)
+	if (dir) {
-		end_dir_add(dir, n, d_wait);
+		end_dir_add(dir, n);
-
+		__d_wake_in_lookup_waiters(target);
+	}
 	if (dentry->d_parent != old_parent)
 		spin_unlock(&dentry->d_parent->d_lock);
 	if (dentry != old_parent)
@@ -3151,7 +3244,7 @@ struct dentry *d_splice_alias_ops(struct inode *inode, struct dentry *dentry,
 	security_d_instantiate(dentry, inode);
 	spin_lock(&inode->i_lock);
 	if (S_ISDIR(inode->i_mode)) {
-		struct dentry *new = __d_find_any_alias(inode);
+		struct dentry *new = __d_find_dir_alias(inode);
 		if (unlikely(new)) {
 			/* The reference to new ensures it remains an alias */
 			spin_unlock(&inode->i_lock);
@@ -3176,6 +3269,12 @@ struct dentry *d_splice_alias_ops(struct inode *inode, struct dentry *dentry,
 				}
 				dput(old_parent);
 			} else {
+				if (unlikely(!hlist_unhashed(&new->d_sib))) {
+					// secondary root getting spliced
+					spin_lock(&new->d_lock);
+					unlink_secondary_root(new);
+					spin_unlock(&new->d_lock);
+				}
 				__d_move(new, dentry, false);
 				write_sequnlock(&rename_lock);
 			}

fs/exportfs/expfs.c

@@ -53,10 +53,10 @@ find_acceptable_alias(struct dentry *result,
 	inode = result->d_inode;
 	spin_lock(&inode->i_lock);
 	for_each_alias(dentry, inode) {
-		dget(dentry);
+		if (!dget_alias_ilocked(dentry))
+			continue;
 		spin_unlock(&inode->i_lock);
-		if (toput)
+		dput(toput);
-			dput(toput);
 		if (dentry != result && acceptable(context, dentry)) {
 			dput(result);
 			return dentry;
@@ -66,8 +66,7 @@ find_acceptable_alias(struct dentry *result,
 	}
 	spin_unlock(&inode->i_lock);
 
-	if (toput)
+	dput(toput);
-		dput(toput);
 	return NULL;
 }
 

fs/file_table.c

@@ -402,6 +402,8 @@ static struct file *alloc_file(const struct path *path, int flags,
 static inline int alloc_path_pseudo(const char *name, struct inode *inode,
 				    struct vfsmount *mnt, struct path *path)
 {
+	if (WARN_ON_ONCE(S_ISDIR(inode->i_mode)))
+		return -EINVAL;
 	path->dentry = d_alloc_pseudo(mnt->mnt_sb, &QSTR(name));
 	if (!path->dentry)
 		return -ENOMEM;

fs/fuse/dir.c

@@ -177,8 +177,8 @@ static void fuse_dentry_tree_work(struct work_struct *work)
 			spin_lock(&fd->dentry->d_lock);
 			/* If dentry is still referenced, let next dput release it */
 			fd->dentry->d_flags |= DCACHE_OP_DELETE;
+			__move_to_shrink_list(fd->dentry, &dispose);
 			spin_unlock(&fd->dentry->d_lock);
-			d_dispose_if_unused(fd->dentry, &dispose);
 			if (need_resched()) {
 				spin_unlock(&dentry_hash[i].lock);
 				cond_resched();

fs/fuse/readdir.c

@@ -164,7 +164,6 @@ static int fuse_direntplus_link(struct file *file,
 	struct inode *dir = d_inode(parent);
 	struct fuse_conn *fc;
 	struct inode *inode;
-	DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
 	int epoch;
 
 	if (!o->nodeid) {
@@ -201,7 +200,7 @@ static int fuse_direntplus_link(struct file *file,
 	dentry = d_lookup(parent, &name);
 	if (!dentry) {
 retry:
-		dentry = d_alloc_parallel(parent, &name, &wq);
+		dentry = d_alloc_parallel(parent, &name);
 		if (IS_ERR(dentry))
 			return PTR_ERR(dentry);
 	}

fs/namei.c

@@ -1896,13 +1896,12 @@ static struct dentry *__lookup_slow(const struct qstr *name,
 {
 	struct dentry *dentry, *old;
 	struct inode *inode = dir->d_inode;
-	DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
 
 	/* Don't go there if it's already dead */
 	if (unlikely(IS_DEADDIR(inode)))
 		return ERR_PTR(-ENOENT);
 again:
-	dentry = d_alloc_parallel(dir, name, &wq);
+	dentry = d_alloc_parallel(dir, name);
 	if (IS_ERR(dentry))
 		return dentry;
 	if (unlikely(!d_in_lookup(dentry))) {
@@ -4413,7 +4412,6 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
 	struct dentry *dentry;
 	int error, create_error = 0;
 	umode_t mode = op->mode;
-	DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
 
 	if (unlikely(IS_DEADDIR(dir_inode)))
 		return ERR_PTR(-ENOENT);
@@ -4422,7 +4420,7 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
 	dentry = d_lookup(dir, &nd->last);
 	for (;;) {
 		if (!dentry) {
-			dentry = d_alloc_parallel(dir, &nd->last, &wq);
+			dentry = d_alloc_parallel(dir, &nd->last);
 			if (IS_ERR(dentry))
 				return dentry;
 		}

fs/nfs/dir.c

@@ -726,7 +726,6 @@ void nfs_prime_dcache(struct dentry *parent, struct nfs_entry *entry,
 		unsigned long dir_verifier)
 {
 	struct qstr filename = QSTR_INIT(entry->name, entry->len);
-	DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
 	struct dentry *dentry;
 	struct dentry *alias;
 	struct inode *inode;
@@ -755,7 +754,7 @@ void nfs_prime_dcache(struct dentry *parent, struct nfs_entry *entry,
 	dentry = d_lookup(parent, &filename);
 again:
 	if (!dentry) {
-		dentry = d_alloc_parallel(parent, &filename, &wq);
+		dentry = d_alloc_parallel(parent, &filename);
 		if (IS_ERR(dentry))
 			return;
 	}
@@ -2106,7 +2105,6 @@ int nfs_atomic_open(struct inode *dir, struct dentry *dentry,
 		    struct file *file, unsigned open_flags,
 		    umode_t mode)
 {
-	DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
 	struct nfs_open_context *ctx;
 	struct dentry *res;
 	struct iattr attr = { .ia_valid = ATTR_OPEN };
@@ -2162,7 +2160,7 @@ int nfs_atomic_open(struct inode *dir, struct dentry *dentry,
 		d_drop(dentry);
 		switched = true;
 		dentry = d_alloc_parallel(dentry->d_parent,
-					  &dentry->d_name, &wq);
+					  &dentry->d_name);
 		if (IS_ERR(dentry))
 			return PTR_ERR(dentry);
 		if (unlikely(!d_in_lookup(dentry)))

fs/nfs/getroot.c

@@ -32,35 +32,6 @@
 
 #define NFSDBG_FACILITY		NFSDBG_CLIENT
 
-/*
- * Set the superblock root dentry.
- * Note that this function frees the inode in case of error.
- */
-static int nfs_superblock_set_dummy_root(struct super_block *sb, struct inode *inode)
-{
-	/* The mntroot acts as the dummy root dentry for this superblock */
-	if (sb->s_root == NULL) {
-		sb->s_root = d_make_root(inode);
-		if (sb->s_root == NULL)
-			return -ENOMEM;
-		ihold(inode);
-		/*
-		 * Ensure that this dentry is invisible to d_find_alias().
-		 * Otherwise, it may be spliced into the tree by
-		 * d_splice_alias if a parent directory from the same
-		 * filesystem gets mounted at a later time.
-		 * This again causes shrink_dcache_for_umount_subtree() to
-		 * Oops, since the test for IS_ROOT() will fail.
-		 */
-		spin_lock(&d_inode(sb->s_root)->i_lock);
-		spin_lock(&sb->s_root->d_lock);
-		hlist_del_init(&sb->s_root->d_alias);
-		spin_unlock(&sb->s_root->d_lock);
-		spin_unlock(&d_inode(sb->s_root)->i_lock);
-	}
-	return 0;
-}
-
 /*
  * get a root dentry from the root filehandle
  */
@@ -99,10 +70,6 @@ int nfs_get_root(struct super_block *s, struct fs_context *fc)
 		goto out_fattr;
 	}
 
-	error = nfs_superblock_set_dummy_root(s, inode);
-	if (error != 0)
-		goto out_fattr;
-
 	/* root dentries normally start off anonymous and get spliced in later
 	 * if the dentry tree reaches them; however if the dentry already
 	 * exists, we'll pick it up at this point and use it as the root
@@ -123,6 +90,8 @@ int nfs_get_root(struct super_block *s, struct fs_context *fc)
 		name = NULL;
 	}
 	spin_unlock(&root->d_lock);
+	if (!s->s_root)
+		s->s_root = dget(root);
 	fc->root = root;
 	if (server->caps & NFS_CAP_SECURITY_LABEL)
 		kflags |= SECURITY_LSM_NATIVE_LABELS;

fs/nfs/unlink.c

@@ -124,7 +124,7 @@ static int nfs_call_unlink(struct dentry *dentry, struct inode *inode, struct nf
 	struct dentry *alias;
 
 	down_read_non_owner(&NFS_I(dir)->rmdir_sem);
-	alias = d_alloc_parallel(dentry->d_parent, &data->args.name, &data->wq);
+	alias = d_alloc_parallel(dentry->d_parent, &data->args.name);
 	if (IS_ERR(alias)) {
 		up_read_non_owner(&NFS_I(dir)->rmdir_sem);
 		return 0;
@@ -185,7 +185,6 @@ nfs_async_unlink(struct dentry *dentry, const struct qstr *name)
 
 	data->cred = get_current_cred();
 	data->res.dir_attr = &data->dir_attr;
-	init_waitqueue_head(&data->wq);
 
 	status = -EBUSY;
 	spin_lock(&dentry->d_lock);

fs/proc/base.c

@@ -2111,8 +2111,7 @@ bool proc_fill_cache(struct file *file, struct dir_context *ctx,
 		goto end_instantiate;
 
 	if (!child) {
-		DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
+		child = d_alloc_parallel(dir, &qname);
-		child = d_alloc_parallel(dir, &qname, &wq);
 		if (IS_ERR(child))
 			goto end_instantiate;
 		if (d_in_lookup(child)) {

fs/proc/proc_sysctl.c

@@ -692,8 +692,7 @@ static bool proc_sys_fill_cache(struct file *file,
 
 	child = d_lookup(dir, &qname);
 	if (!child) {
-		DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
+		child = d_alloc_parallel(dir, &qname);
-		child = d_alloc_parallel(dir, &qname, &wq);
 		if (IS_ERR(child))
 			return false;
 		if (d_in_lookup(child)) {

fs/smb/client/readdir.c

@@ -73,7 +73,6 @@ cifs_prime_dcache(struct dentry *parent, struct qstr *name,
 	struct cifs_sb_info *cifs_sb = CIFS_SB(sb);
 	bool posix = cifs_sb_master_tcon(cifs_sb)->posix_extensions;
 	bool reparse_need_reval = false;
-	DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
 	int rc;
 
 	cifs_dbg(FYI, "%s: for %s\n", __func__, name->name);
@@ -105,7 +104,7 @@ retry:
 		    (fattr->cf_flags & CIFS_FATTR_NEED_REVAL))
 			return;
 
-		dentry = d_alloc_parallel(parent, name, &wq);
+		dentry = d_alloc_parallel(parent, name);
 	}
 	if (IS_ERR(dentry))
 		return;

fs/super.c

@@ -359,6 +359,7 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
 		s->s_iflags |= SB_I_NODEV;
 	INIT_HLIST_NODE(&s->s_instances);
 	INIT_HLIST_BL_HEAD(&s->s_roots);
+	spin_lock_init(&s->s_roots_lock);
 	mutex_init(&s->s_sync_lock);
 	INIT_LIST_HEAD(&s->s_inodes);
 	spin_lock_init(&s->s_inode_list_lock);

include/linux/dcache.h

@@ -116,10 +116,7 @@ struct dentry {
 					 * possible!
 					 */
 
-	union {
+	struct list_head d_lru;		/* LRU list */
-		struct list_head d_lru;		/* LRU list */
-		wait_queue_head_t *d_wait;	/* in-lookup ones only */
-	};
 	struct hlist_node d_sib;	/* child of parent list */
 	struct hlist_head d_children;	/* our children */
 	/*
@@ -210,6 +207,9 @@ enum dentry_flags {
 	DCACHE_REFERENCED		= BIT(6),	/* Recently used, don't discard. */
 	DCACHE_DONTCACHE		= BIT(7),	/* Purge from memory on final dput() */
 	DCACHE_CANT_MOUNT		= BIT(8),
+	DCACHE_LOOKUP_WAITERS		= BIT(9),	/* A thread is waiting for
+							 * PAR_LOOKUP to clear
+							 */
 	DCACHE_SHRINK_LIST		= BIT(10),
 	DCACHE_OP_WEAK_REVALIDATE	= BIT(11),
 	/*
@@ -256,8 +256,7 @@ extern void d_delete(struct dentry *);
 /* allocate/de-allocate */
 extern struct dentry * d_alloc(struct dentry *, const struct qstr *);
 extern struct dentry * d_alloc_anon(struct super_block *);
-extern struct dentry * d_alloc_parallel(struct dentry *, const struct qstr *,
+extern struct dentry * d_alloc_parallel(struct dentry *, const struct qstr *);
-					wait_queue_head_t *);
 extern struct dentry * d_splice_alias(struct inode *, struct dentry *);
 /* weird procfs mess; *NOT* exported */
 extern struct dentry * d_splice_alias_ops(struct inode *, struct dentry *,
@@ -281,7 +280,7 @@ extern void d_tmpfile(struct file *, struct inode *);
 
 extern struct dentry *d_find_alias(struct inode *);
 extern void d_prune_aliases(struct inode *);
-extern void d_dispose_if_unused(struct dentry *, struct list_head *);
+extern bool __move_to_shrink_list(struct dentry *, struct list_head *);
 extern void shrink_dentry_list(struct list_head *);
 
 extern struct dentry *d_find_alias_rcu(struct inode *);
@@ -366,6 +365,24 @@ static inline struct dentry *dget(struct dentry *dentry)
 	return dentry;
 }
 
+/* dentry->d_inode->i_lock must be held by caller */
+static inline bool dget_alias_ilocked(struct dentry *dentry)
+{
+	if (likely(!(READ_ONCE(dentry->d_flags) & DCACHE_NORCU))) {
+		lockref_get(&dentry->d_lockref);
+		return true;
+	}
+	// NORCU dentries with zero refcount MUST NOT be grabbed
+	spin_lock(&dentry->d_lock);
+	if (dentry->d_lockref.count > 0) {
+		dget_dlock(dentry);
+		spin_unlock(&dentry->d_lock);
+		return true;
+	}
+	spin_unlock(&dentry->d_lock);
+	return false;
+}
+
 extern struct dentry *dget_parent(struct dentry *dentry);
 
 /**

include/linux/fs/super_types.h

@@ -162,7 +162,8 @@ struct super_block {
 	struct unicode_map			*s_encoding;
 	__u16					s_encoding_flags;
 #endif
-	struct hlist_bl_head			s_roots;	/* alternate root dentries for NFS */
+	struct hlist_head			s_roots;	/* alternate root dentries for NFS */
+	spinlock_t				s_roots_lock;
 	struct mount				*s_mounts;	/* list of mounts; _not_ for fs use */
 	struct block_device			*s_bdev;	/* can go away once we use an accessor for @s_bdev_file */
 	struct file				*s_bdev_file;

include/linux/nfs_xdr.h

@@ -1745,7 +1745,6 @@ struct nfs_unlinkdata {
 	struct nfs_removeargs args;
 	struct nfs_removeres res;
 	struct dentry *dentry;
-	wait_queue_head_t wq;
 	const struct cred *cred;
 	struct nfs_fattr dir_attr;
 	long timeout;