Re: [linus:master] [eventfs] 852e46e239: BUG:unable_to_handle_page_fault_for_address

[Linus Torvalds <torvalds@linux-foundation.org>]

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On Mon, 29 Jan 2024 at 17:50, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> So what I propose is that
>
>  - ei->dentry and ei->d_children[] need to die. Really. They are
> buggy. There is no way to save them. There never was.
>
>  - but we *can* introduce a new 'ei->events_dir' pointer that is
> *only* set by eventfs_create_events_dir(), and which is stable exactly
> because that function also does a dget() on it, so now the dentry will
> actually continue to exist reliably
>
> I think that works.

Well, it doesn't. I don't see where the bug is, but since Al is now
aware of the thread, maybe when he wakes up he will tell me where I've
gone wrong.

In the meantime I did do the pending tracefs pull, so the series has
changed a bit, and this is the rebased series on top of my current
public git tree.

It is still broken wrt 'events' directories. You don't even need the
"create, delete, create" sequence that Steven pointed out, just a
plain sequence of

 # cd /sys/kernel/tracing
 # ls events/kprobes/
 # echo 'p:sched schedule' >> kprobe_events

messes up - ie it's enough to just have 'lookup' create a negative
dentry by trying to look up 'events/kprobes/' before actually trying
to create that kprobe_events.

But I've been staring at this code for too long, so I'm posting this
just as a "it's broken, but _something_ like this", because I'm taking
a break from looking at this.

I'll get back to it tomorrow, but I hope that Al will show me the
error of my ways.

              Linus

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From 6763ac4af7ccc0c97fb5f7c98d0c8ae1289ec0fe Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 29 Jan 2024 18:49:42 -0800
Subject: [PATCH 5/5] eventfs: get rid of dentry pointers without refcounts

The eventfs inode had pointers to dentries (and child dentries) without
actually holding a refcount on said pointer.  That is fundamentally
broken, and while eventfs tried to then maintain coherence with dentries
going away by hooking into the '.d_iput' callback, that doesn't actually
work since it's not ordered wrt lookups.

There were two reasonms why eventfs tried to keep a pointer to a dentry:

 - the creation of a 'events' directory would actually have a stable
   dentry pointer that it created with tracefs_start_creating().

   And it needed that dentry when tearing it all down again in
   eventfs_remove_events_dir().

   This use is actually ok, because the special top-level events
   directory dentries are actually stable, not just a temporary cache of
   the eventfs data structures.

 - the 'eventfs_inode' (aka ei) needs to stay around as long as there
   are dentries that refer to it.

   It then used these dentry pointers as a replacement for doing
   reference counting: it would try to make sure that there was only
   ever one dentry associated with an event_inode, and keep a child
   dentry array around to see which dentries might still refer to the
   parent ei.

This gets rid of the invalid dentry pointer use, and renames the one
valid case to a different name to make it clear that it's not just any
random dentry.

The magic child dentry array that is kind of a "reverse reference list"
is simply replaced by having child dentries take a ref to the ei.  As
does the directory dentries.  That makes the broken use case go away.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
 fs/tracefs/event_inode.c | 245 ++++++++++++---------------------------
 fs/tracefs/internal.h    |   9 +-
 2 files changed, 80 insertions(+), 174 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index 1d0102bfd7da..a37db0dac302 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -62,6 +62,34 @@ enum {
 
 #define EVENTFS_MODE_MASK	(EVENTFS_SAVE_MODE - 1)
 
+/*
+ * eventfs_inode reference count management.
+ *
+ * NOTE! We count only references from dentries, in the
+ * form 'dentry->d_fsdata'. There are also references from
+ * directory inodes ('ti->private'), but the dentry reference
+ * count is always a superset of the inode reference count.
+ */
+static void release_ei(struct kref *ref)
+{
+	struct eventfs_inode *ei = container_of(ref, struct eventfs_inode, kref);
+	kfree(ei->entry_attrs);
+	kfree(ei);
+}
+
+static inline void put_ei(struct eventfs_inode *ei)
+{
+	if (ei)
+		kref_put(&ei->kref, release_ei);
+}
+
+static inline struct eventfs_inode *get_ei(struct eventfs_inode *ei)
+{
+	if (ei)
+		kref_get(&ei->kref);
+	return ei;
+}
+
 static struct dentry *eventfs_root_lookup(struct inode *dir,
 					  struct dentry *dentry,
 					  unsigned int flags);
@@ -289,7 +317,8 @@ static void update_inode_attr(struct dentry *dentry, struct inode *inode,
  * directory. The inode.i_private pointer will point to @data in the open()
  * call.
  */
-static struct dentry *lookup_file(struct dentry *dentry,
+static struct dentry *lookup_file(struct eventfs_inode *parent_ei,
+				  struct dentry *dentry,
 				  umode_t mode,
 				  struct eventfs_attr *attr,
 				  void *data,
@@ -302,11 +331,11 @@ static struct dentry *lookup_file(struct dentry *dentry,
 		mode |= S_IFREG;
 
 	if (WARN_ON_ONCE(!S_ISREG(mode)))
-		return NULL;
+		return ERR_PTR(-EIO);
 
 	inode = tracefs_get_inode(dentry->d_sb);
 	if (unlikely(!inode))
-		return eventfs_failed_creating(dentry);
+		return ERR_PTR(-ENOMEM);
 
 	/* If the user updated the directory's attributes, use them */
 	update_inode_attr(dentry, inode, attr, mode);
@@ -322,9 +351,12 @@ static struct dentry *lookup_file(struct dentry *dentry,
 	ti->flags = TRACEFS_EVENT_INODE;
 	ti->private = NULL;			// Directories have 'ei', files not
 
+	// Files have their parent's ei as their fsdata
+	dentry->d_fsdata = get_ei(parent_ei);
+
 	d_add(dentry, inode);
 	fsnotify_create(dentry->d_parent->d_inode, dentry);
-	return eventfs_end_creating(dentry);
+	return NULL;
 };
 
 /**
@@ -343,7 +375,7 @@ static struct dentry *lookup_dir_entry(struct dentry *dentry,
 
 	inode = tracefs_get_inode(dentry->d_sb);
 	if (unlikely(!inode))
-		return eventfs_failed_creating(dentry);
+		return ERR_PTR(-ENOMEM);
 
 	/* If the user updated the directory's attributes, use them */
 	update_inode_attr(dentry, inode, &ei->attr,
@@ -359,24 +391,28 @@ static struct dentry *lookup_dir_entry(struct dentry *dentry,
 	ti->flags = TRACEFS_EVENT_INODE;
 	ti->private = ei;
 
-	dentry->d_fsdata = ei;
-        ei->dentry = dentry;	// Remove me!
+	dentry->d_fsdata = get_ei(ei);
 
 	inc_nlink(inode);
 	d_add(dentry, inode);
 	inc_nlink(dentry->d_parent->d_inode);
 	fsnotify_mkdir(dentry->d_parent->d_inode, dentry);
-	return eventfs_end_creating(dentry);
+	return NULL;
 }
 
-static void free_ei(struct eventfs_inode *ei)
+static inline struct eventfs_inode *alloc_ei(const char *name)
 {
-	kfree_const(ei->name);
-	kfree(ei->d_children);
-	kfree(ei->entry_attrs);
-	kfree(ei);
+	int namesize = strlen(name) + 1;
+	struct eventfs_inode *ei = kzalloc(sizeof(*ei) + namesize, GFP_KERNEL);
+
+	if (ei) {
+		memcpy((char *)ei->name, name, namesize);
+		kref_init(&ei->kref);
+	}
+	return ei;
 }
 
+
 /**
  * eventfs_set_ei_status_free - remove the dentry reference from an eventfs_inode
  * @ti: the tracefs_inode of the dentry
@@ -387,38 +423,20 @@ static void free_ei(struct eventfs_inode *ei)
 void eventfs_set_ei_status_free(struct tracefs_inode *ti, struct dentry *dentry)
 {
 	struct eventfs_inode *ei;
-	int i;
 
 	mutex_lock(&eventfs_mutex);
-
 	ei = dentry->d_fsdata;
-	if (!ei)
-		goto out;
-
-	/* This could belong to one of the files of the ei */
-	if (ei->dentry != dentry) {
-		for (i = 0; i < ei->nr_entries; i++) {
-			if (ei->d_children[i] == dentry)
-				break;
-		}
-		if (WARN_ON_ONCE(i == ei->nr_entries))
-			goto out;
-		ei->d_children[i] = NULL;
-	} else if (ei->is_freed) {
-		free_ei(ei);
-	} else {
-		ei->dentry = NULL;
+	if (ei) {
+		dentry->d_fsdata = NULL;
+		put_ei(ei);
 	}
-
-	dentry->d_fsdata = NULL;
- out:
 	mutex_unlock(&eventfs_mutex);
 }
 
 /**
  * lookup_file_dentry - create a dentry for a file of an eventfs_inode
  * @ei: the eventfs_inode that the file will be created under
- * @idx: the index into the d_children[] of the @ei
+ * @idx: the index into the entry_attrs[] of the @ei
  * @parent: The parent dentry of the created file.
  * @name: The name of the file to create
  * @mode: The mode of the file.
@@ -435,17 +453,11 @@ lookup_file_dentry(struct dentry *dentry,
 		   const struct file_operations *fops)
 {
 	struct eventfs_attr *attr = NULL;
-	struct dentry **e_dentry = &ei->d_children[idx];
 
 	if (ei->entry_attrs)
 		attr = &ei->entry_attrs[idx];
 
-	dentry->d_fsdata = ei;		// NOTE: ei of _parent_
-	lookup_file(dentry, mode, attr, data, fops);
-
-	*e_dentry = dentry;	// Remove me
-
-	return dentry;
+	return lookup_file(ei, dentry, mode, attr, data, fops);
 }
 
 /**
@@ -466,6 +478,7 @@ static struct dentry *eventfs_root_lookup(struct inode *dir,
 	struct tracefs_inode *ti;
 	struct eventfs_inode *ei;
 	const char *name = dentry->d_name.name;
+	struct dentry *result;
 
 	ti = get_tracefs(dir);
 	if (!(ti->flags & TRACEFS_EVENT_INODE))
@@ -482,7 +495,7 @@ static struct dentry *eventfs_root_lookup(struct inode *dir,
 			continue;
 		if (ei_child->is_freed)
 			goto enoent;
-		lookup_dir_entry(dentry, ei, ei_child);
+		result = lookup_dir_entry(dentry, ei, ei_child);
 		goto out;
 	}
 
@@ -499,17 +512,18 @@ static struct dentry *eventfs_root_lookup(struct inode *dir,
 		if (entry->callback(name, &mode, &data, &fops) <= 0)
 			goto enoent;
 
-		lookup_file_dentry(dentry, ei, i, mode, data, fops);
+		result = lookup_file_dentry(dentry, ei, i, mode, data, fops);
 		goto out;
 	}
 
  enoent:
 	/* Nothing found? */
 	d_add(dentry, NULL);
+	result = NULL;
 
  out:
 	mutex_unlock(&eventfs_mutex);
-	return NULL;
+	return result;
 }
 
 /*
@@ -659,25 +673,10 @@ struct eventfs_inode *eventfs_create_dir(const char *name, struct eventfs_inode
 	if (!parent)
 		return ERR_PTR(-EINVAL);
 
-	ei = kzalloc(sizeof(*ei), GFP_KERNEL);
+	ei = alloc_ei(name);
 	if (!ei)
 		return ERR_PTR(-ENOMEM);
 
-	ei->name = kstrdup_const(name, GFP_KERNEL);
-	if (!ei->name) {
-		kfree(ei);
-		return ERR_PTR(-ENOMEM);
-	}
-
-	if (size) {
-		ei->d_children = kcalloc(size, sizeof(*ei->d_children), GFP_KERNEL);
-		if (!ei->d_children) {
-			kfree_const(ei->name);
-			kfree(ei);
-			return ERR_PTR(-ENOMEM);
-		}
-	}
-
 	ei->entries = entries;
 	ei->nr_entries = size;
 	ei->data = data;
@@ -691,7 +690,7 @@ struct eventfs_inode *eventfs_create_dir(const char *name, struct eventfs_inode
 
 	/* Was the parent freed? */
 	if (list_empty(&ei->list)) {
-		free_ei(ei);
+		put_ei(ei);
 		ei = NULL;
 	}
 	return ei;
@@ -726,28 +725,20 @@ struct eventfs_inode *eventfs_create_events_dir(const char *name, struct dentry
 	if (IS_ERR(dentry))
 		return ERR_CAST(dentry);
 
-	ei = kzalloc(sizeof(*ei), GFP_KERNEL);
+	ei = alloc_ei(name);
 	if (!ei)
-		goto fail_ei;
+		goto fail;
 
 	inode = tracefs_get_inode(dentry->d_sb);
 	if (unlikely(!inode))
 		goto fail;
 
-	if (size) {
-		ei->d_children = kcalloc(size, sizeof(*ei->d_children), GFP_KERNEL);
-		if (!ei->d_children)
-			goto fail;
-	}
-
-	ei->dentry = dentry;
+	// Note: we have a ref to the dentry from tracefs_start_creating()
+	ei->events_dir = dentry;
 	ei->entries = entries;
 	ei->nr_entries = size;
 	ei->is_events = 1;
 	ei->data = data;
-	ei->name = kstrdup_const(name, GFP_KERNEL);
-	if (!ei->name)
-		goto fail;
 
 	/* Save the ownership of this directory */
 	uid = d_inode(dentry->d_parent)->i_uid;
@@ -778,7 +769,7 @@ struct eventfs_inode *eventfs_create_events_dir(const char *name, struct dentry
 	inode->i_op = &eventfs_root_dir_inode_operations;
 	inode->i_fop = &eventfs_file_operations;
 
-	dentry->d_fsdata = ei;
+	dentry->d_fsdata = get_ei(ei);
 
 	/* directory inodes start off with i_nlink == 2 (for "." entry) */
 	inc_nlink(inode);
@@ -790,72 +781,11 @@ struct eventfs_inode *eventfs_create_events_dir(const char *name, struct dentry
 	return ei;
 
  fail:
-	kfree(ei->d_children);
-	kfree(ei);
- fail_ei:
+	put_ei(ei);
 	tracefs_failed_creating(dentry);
 	return ERR_PTR(-ENOMEM);
 }
 
-static LLIST_HEAD(free_list);
-
-static void eventfs_workfn(struct work_struct *work)
-{
-        struct eventfs_inode *ei, *tmp;
-        struct llist_node *llnode;
-
-	llnode = llist_del_all(&free_list);
-        llist_for_each_entry_safe(ei, tmp, llnode, llist) {
-		/* This dput() matches the dget() from unhook_dentry() */
-		for (int i = 0; i < ei->nr_entries; i++) {
-			if (ei->d_children[i])
-				dput(ei->d_children[i]);
-		}
-		/* This should only get here if it had a dentry */
-		if (!WARN_ON_ONCE(!ei->dentry))
-			dput(ei->dentry);
-        }
-}
-
-static DECLARE_WORK(eventfs_work, eventfs_workfn);
-
-static void free_rcu_ei(struct rcu_head *head)
-{
-	struct eventfs_inode *ei = container_of(head, struct eventfs_inode, rcu);
-
-	if (ei->dentry) {
-		/* Do not free the ei until all references of dentry are gone */
-		if (llist_add(&ei->llist, &free_list))
-			queue_work(system_unbound_wq, &eventfs_work);
-		return;
-	}
-
-	/* If the ei doesn't have a dentry, neither should its children */
-	for (int i = 0; i < ei->nr_entries; i++) {
-		WARN_ON_ONCE(ei->d_children[i]);
-	}
-
-	free_ei(ei);
-}
-
-static void unhook_dentry(struct dentry *dentry)
-{
-	if (!dentry)
-		return;
-	/*
-	 * Need to add a reference to the dentry that is expected by
-	 * simple_recursive_removal(), which will include a dput().
-	 */
-	dget(dentry);
-
-	/*
-	 * Also add a reference for the dput() in eventfs_workfn().
-	 * That is required as that dput() will free the ei after
-	 * the SRCU grace period is over.
-	 */
-	dget(dentry);
-}
-
 /**
  * eventfs_remove_rec - remove eventfs dir or file from list
  * @ei: eventfs_inode to be removed.
@@ -868,8 +798,6 @@ static void eventfs_remove_rec(struct eventfs_inode *ei, int level)
 {
 	struct eventfs_inode *ei_child;
 
-	if (!ei)
-		return;
 	/*
 	 * Check recursion depth. It should never be greater than 3:
 	 * 0 - events/
@@ -881,28 +809,12 @@ static void eventfs_remove_rec(struct eventfs_inode *ei, int level)
 		return;
 
 	/* search for nested folders or files */
-	list_for_each_entry_srcu(ei_child, &ei->children, list,
-				 lockdep_is_held(&eventfs_mutex)) {
-		/* Children only have dentry if parent does */
-		WARN_ON_ONCE(ei_child->dentry && !ei->dentry);
+	list_for_each_entry(ei_child, &ei->children, list)
 		eventfs_remove_rec(ei_child, level + 1);
-	}
-
 
 	ei->is_freed = 1;
-
-	for (int i = 0; i < ei->nr_entries; i++) {
-		if (ei->d_children[i]) {
-			/* Children only have dentry if parent does */
-			WARN_ON_ONCE(!ei->dentry);
-			unhook_dentry(ei->d_children[i]);
-		}
-	}
-
-	unhook_dentry(ei->dentry);
-
-	list_del_rcu(&ei->list);
-	call_srcu(&eventfs_srcu, &ei->rcu, free_rcu_ei);
+	list_del(&ei->list);
+	put_ei(ei);
 }
 
 /**
@@ -913,22 +825,12 @@ static void eventfs_remove_rec(struct eventfs_inode *ei, int level)
  */
 void eventfs_remove_dir(struct eventfs_inode *ei)
 {
-	struct dentry *dentry;
-
 	if (!ei)
 		return;
 
 	mutex_lock(&eventfs_mutex);
-	dentry = ei->dentry;
 	eventfs_remove_rec(ei, 0);
 	mutex_unlock(&eventfs_mutex);
-
-	/*
-	 * If any of the ei children has a dentry, then the ei itself
-	 * must have a dentry.
-	 */
-	if (dentry)
-		simple_recursive_removal(dentry, NULL);
 }
 
 /**
@@ -941,7 +843,11 @@ void eventfs_remove_events_dir(struct eventfs_inode *ei)
 {
 	struct dentry *dentry;
 
-	dentry = ei->dentry;
+	dentry = ei->events_dir;
+	if (!dentry)
+		return;
+
+	ei->events_dir = NULL;
 	eventfs_remove_dir(ei);
 
 	/*
@@ -951,5 +857,6 @@ void eventfs_remove_events_dir(struct eventfs_inode *ei)
 	 * sticks around while the other ei->dentry are created
 	 * and destroyed dynamically.
 	 */
+	simple_recursive_removal(dentry, NULL);
 	dput(dentry);
 }
diff --git a/fs/tracefs/internal.h b/fs/tracefs/internal.h
index 8f38740bfb5b..72db3bdc4dfb 100644
--- a/fs/tracefs/internal.h
+++ b/fs/tracefs/internal.h
@@ -34,8 +34,7 @@ struct eventfs_attr {
  * @entries:	the array of entries representing the files in the directory
  * @name:	the name of the directory to create
  * @children:	link list into the child eventfs_inode
- * @dentry:     the dentry of the directory
- * @d_children: The array of dentries to represent the files when created
+ * @events_dir: the dentry of the events directory
  * @entry_attrs: Saved mode and ownership of the @d_children
  * @attr:	Saved mode and ownership of eventfs_inode itself
  * @data:	The private data to pass to the callbacks
@@ -44,12 +43,11 @@ struct eventfs_attr {
  * @nr_entries: The number of items in @entries
  */
 struct eventfs_inode {
+	struct kref			kref;
 	struct list_head		list;
 	const struct eventfs_entry	*entries;
-	const char			*name;
 	struct list_head		children;
-	struct dentry			*dentry; /* Check is_freed to access */
-	struct dentry			**d_children;
+	struct dentry			*events_dir;
 	struct eventfs_attr		*entry_attrs;
 	struct eventfs_attr		attr;
 	void				*data;
@@ -66,6 +64,7 @@ struct eventfs_inode {
 		struct llist_node	llist;
 		struct rcu_head		rcu;
 	};
+	const char name[];
 };
 
 static inline struct tracefs_inode *get_tracefs(const struct inode *inode)
-- 
2.43.0.5.g38fb137bdb


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From b2ee7a777a8201aed790ef8cf8fd1f10f4f08aa3 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 29 Jan 2024 12:25:53 -0800
Subject: [PATCH 3/5] tracefs: dentry lookup crapectomy

The dentry lookup for eventfs files was very broken, and had lots of
signs of the old situation where the filesystem names were all created
statically in the dentry tree, rather than being looked up dynamically
based on the eventfs data structures.

You could see it in the naming - how it claimed to "create" dentries
rather than just look up the dentries that were given it.

You could see it in various nonsensical and very incorrect operations,
like using "simple_lookup()" on the dentries that were passed in, which
only results in those dentries becoming negative dentries.  Which meant
that any other lookup would possibly return ENOENT if it saw that
negative dentry before the data rwas then later filled in.

You could see it in the immesnse amount of nonsensical code that didn't
actually just do lookups.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
 fs/tracefs/event_inode.c | 275 ++++++++-------------------------------
 1 file changed, 52 insertions(+), 223 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index c0d977e6c0f2..ad11063bdd53 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -230,7 +230,6 @@ static struct eventfs_inode *eventfs_find_events(struct dentry *dentry)
 {
 	struct eventfs_inode *ei;
 
-	mutex_lock(&eventfs_mutex);
 	do {
 		// The parent is stable because we do not do renames
 		dentry = dentry->d_parent;
@@ -247,7 +246,6 @@ static struct eventfs_inode *eventfs_find_events(struct dentry *dentry)
 		}
 		// Walk upwards until you find the events inode
 	} while (!ei->is_events);
-	mutex_unlock(&eventfs_mutex);
 
 	update_top_events_attr(ei, dentry->d_sb);
 
@@ -280,11 +278,10 @@ static void update_inode_attr(struct dentry *dentry, struct inode *inode,
 }
 
 /**
- * create_file - create a file in the tracefs filesystem
- * @name: the name of the file to create.
+ * lookup_file - look up a file in the tracefs filesystem
+ * @dentry: the dentry to look up
  * @mode: the permission that the file should have.
  * @attr: saved attributes changed by user
- * @parent: parent dentry for this file.
  * @data: something that the caller will want to get to later on.
  * @fop: struct file_operations that should be used for this file.
  *
@@ -292,13 +289,13 @@ static void update_inode_attr(struct dentry *dentry, struct inode *inode,
  * directory. The inode.i_private pointer will point to @data in the open()
  * call.
  */
-static struct dentry *create_file(const char *name, umode_t mode,
+static struct dentry *lookup_file(struct dentry *dentry,
+				  umode_t mode,
 				  struct eventfs_attr *attr,
-				  struct dentry *parent, void *data,
+				  void *data,
 				  const struct file_operations *fop)
 {
 	struct tracefs_inode *ti;
-	struct dentry *dentry;
 	struct inode *inode;
 
 	if (!(mode & S_IFMT))
@@ -307,12 +304,6 @@ static struct dentry *create_file(const char *name, umode_t mode,
 	if (WARN_ON_ONCE(!S_ISREG(mode)))
 		return NULL;
 
-	WARN_ON_ONCE(!parent);
-	dentry = eventfs_start_creating(name, parent);
-
-	if (IS_ERR(dentry))
-		return dentry;
-
 	inode = tracefs_get_inode(dentry->d_sb);
 	if (unlikely(!inode))
 		return eventfs_failed_creating(dentry);
@@ -331,29 +322,25 @@ static struct dentry *create_file(const char *name, umode_t mode,
 	ti->flags = TRACEFS_EVENT_INODE;
 	ti->private = NULL;			// Directories have 'ei', files not
 
-	d_instantiate(dentry, inode);
+	d_add(dentry, inode);
 	fsnotify_create(dentry->d_parent->d_inode, dentry);
 	return eventfs_end_creating(dentry);
 };
 
 /**
- * create_dir - create a dir in the tracefs filesystem
+ * lookup_dir_entry - look up a dir in the tracefs filesystem
+ * @dentry: the directory to look up
  * @ei: the eventfs_inode that represents the directory to create
- * @parent: parent dentry for this file.
  *
- * This function will create a dentry for a directory represented by
+ * This function will look up a dentry for a directory represented by
  * a eventfs_inode.
  */
-static struct dentry *create_dir(struct eventfs_inode *ei, struct dentry *parent)
+static struct dentry *lookup_dir_entry(struct dentry *dentry,
+	struct eventfs_inode *pei, struct eventfs_inode *ei)
 {
 	struct tracefs_inode *ti;
-	struct dentry *dentry;
 	struct inode *inode;
 
-	dentry = eventfs_start_creating(ei->name, parent);
-	if (IS_ERR(dentry))
-		return dentry;
-
 	inode = tracefs_get_inode(dentry->d_sb);
 	if (unlikely(!inode))
 		return eventfs_failed_creating(dentry);
@@ -372,8 +359,11 @@ static struct dentry *create_dir(struct eventfs_inode *ei, struct dentry *parent
 	ti->flags = TRACEFS_EVENT_INODE;
 	ti->private = ei;
 
+	dentry->d_fsdata = ei;
+        ei->dentry = dentry;	// Remove me!
+
 	inc_nlink(inode);
-	d_instantiate(dentry, inode);
+	d_add(dentry, inode);
 	inc_nlink(dentry->d_parent->d_inode);
 	fsnotify_mkdir(dentry->d_parent->d_inode, dentry);
 	return eventfs_end_creating(dentry);
@@ -426,7 +416,7 @@ void eventfs_set_ei_status_free(struct tracefs_inode *ti, struct dentry *dentry)
 }
 
 /**
- * create_file_dentry - create a dentry for a file of an eventfs_inode
+ * lookup_file_dentry - create a dentry for a file of an eventfs_inode
  * @ei: the eventfs_inode that the file will be created under
  * @idx: the index into the d_children[] of the @ei
  * @parent: The parent dentry of the created file.
@@ -439,157 +429,21 @@ void eventfs_set_ei_status_free(struct tracefs_inode *ti, struct dentry *dentry)
  * address located at @e_dentry.
  */
 static struct dentry *
-create_file_dentry(struct eventfs_inode *ei, int idx,
-		   struct dentry *parent, const char *name, umode_t mode, void *data,
+lookup_file_dentry(struct dentry *dentry,
+		   struct eventfs_inode *ei, int idx,
+		   umode_t mode, void *data,
 		   const struct file_operations *fops)
 {
 	struct eventfs_attr *attr = NULL;
 	struct dentry **e_dentry = &ei->d_children[idx];
-	struct dentry *dentry;
 
-	WARN_ON_ONCE(!inode_is_locked(parent->d_inode));
-
-	mutex_lock(&eventfs_mutex);
-	if (ei->is_freed) {
-		mutex_unlock(&eventfs_mutex);
-		return NULL;
-	}
-	/* If the e_dentry already has a dentry, use it */
-	if (*e_dentry) {
-		dget(*e_dentry);
-		mutex_unlock(&eventfs_mutex);
-		return *e_dentry;
-	}
-
-	/* ei->entry_attrs are protected by SRCU */
 	if (ei->entry_attrs)
 		attr = &ei->entry_attrs[idx];
 
-	mutex_unlock(&eventfs_mutex);
+	dentry->d_fsdata = ei;		// NOTE: ei of _parent_
+	lookup_file(dentry, mode, attr, data, fops);
 
-	dentry = create_file(name, mode, attr, parent, data, fops);
-
-	mutex_lock(&eventfs_mutex);
-
-	if (IS_ERR_OR_NULL(dentry)) {
-		/*
-		 * When the mutex was released, something else could have
-		 * created the dentry for this e_dentry. In which case
-		 * use that one.
-		 *
-		 * If ei->is_freed is set, the e_dentry is currently on its
-		 * way to being freed, don't return it. If e_dentry is NULL
-		 * it means it was already freed.
-		 */
-		if (ei->is_freed) {
-			dentry = NULL;
-		} else {
-			dentry = *e_dentry;
-			dget(dentry);
-		}
-		mutex_unlock(&eventfs_mutex);
-		return dentry;
-	}
-
-	if (!*e_dentry && !ei->is_freed) {
-		*e_dentry = dentry;
-		dentry->d_fsdata = ei;
-	} else {
-		/*
-		 * Should never happen unless we get here due to being freed.
-		 * Otherwise it means two dentries exist with the same name.
-		 */
-		WARN_ON_ONCE(!ei->is_freed);
-		dentry = NULL;
-	}
-	mutex_unlock(&eventfs_mutex);
-
-	return dentry;
-}
-
-/**
- * eventfs_post_create_dir - post create dir routine
- * @ei: eventfs_inode of recently created dir
- *
- * Map the meta-data of files within an eventfs dir to their parent dentry
- */
-static void eventfs_post_create_dir(struct eventfs_inode *ei)
-{
-	struct eventfs_inode *ei_child;
-
-	lockdep_assert_held(&eventfs_mutex);
-
-	/* srcu lock already held */
-	/* fill parent-child relation */
-	list_for_each_entry_srcu(ei_child, &ei->children, list,
-				 srcu_read_lock_held(&eventfs_srcu)) {
-		ei_child->d_parent = ei->dentry;
-	}
-}
-
-/**
- * create_dir_dentry - Create a directory dentry for the eventfs_inode
- * @pei: The eventfs_inode parent of ei.
- * @ei: The eventfs_inode to create the directory for
- * @parent: The dentry of the parent of this directory
- *
- * This creates and attaches a directory dentry to the eventfs_inode @ei.
- */
-static struct dentry *
-create_dir_dentry(struct eventfs_inode *pei, struct eventfs_inode *ei,
-		  struct dentry *parent)
-{
-	struct dentry *dentry = NULL;
-
-	WARN_ON_ONCE(!inode_is_locked(parent->d_inode));
-
-	mutex_lock(&eventfs_mutex);
-	if (pei->is_freed || ei->is_freed) {
-		mutex_unlock(&eventfs_mutex);
-		return NULL;
-	}
-	if (ei->dentry) {
-		/* If the eventfs_inode already has a dentry, use it */
-		dentry = ei->dentry;
-		dget(dentry);
-		mutex_unlock(&eventfs_mutex);
-		return dentry;
-	}
-	mutex_unlock(&eventfs_mutex);
-
-	dentry = create_dir(ei, parent);
-
-	mutex_lock(&eventfs_mutex);
-
-	if (IS_ERR_OR_NULL(dentry) && !ei->is_freed) {
-		/*
-		 * When the mutex was released, something else could have
-		 * created the dentry for this e_dentry. In which case
-		 * use that one.
-		 *
-		 * If ei->is_freed is set, the e_dentry is currently on its
-		 * way to being freed.
-		 */
-		dentry = ei->dentry;
-		if (dentry)
-			dget(dentry);
-		mutex_unlock(&eventfs_mutex);
-		return dentry;
-	}
-
-	if (!ei->dentry && !ei->is_freed) {
-		ei->dentry = dentry;
-		eventfs_post_create_dir(ei);
-		dentry->d_fsdata = ei;
-	} else {
-		/*
-		 * Should never happen unless we get here due to being freed.
-		 * Otherwise it means two dentries exist with the same name.
-		 */
-		WARN_ON_ONCE(!ei->is_freed);
-		dentry = NULL;
-	}
-	mutex_unlock(&eventfs_mutex);
+	*e_dentry = dentry;	// Remove me
 
 	return dentry;
 }
@@ -608,79 +462,54 @@ static struct dentry *eventfs_root_lookup(struct inode *dir,
 					  struct dentry *dentry,
 					  unsigned int flags)
 {
-	const struct file_operations *fops;
-	const struct eventfs_entry *entry;
 	struct eventfs_inode *ei_child;
 	struct tracefs_inode *ti;
 	struct eventfs_inode *ei;
-	struct dentry *ei_dentry = NULL;
-	struct dentry *ret = NULL;
-	struct dentry *d;
 	const char *name = dentry->d_name.name;
-	umode_t mode;
-	void *data;
-	int idx;
-	int i;
-	int r;
 
 	ti = get_tracefs(dir);
 	if (!(ti->flags & TRACEFS_EVENT_INODE))
-		return NULL;
+		return ERR_PTR(-EIO);
 
-	/* Grab srcu to prevent the ei from going away */
-	idx = srcu_read_lock(&eventfs_srcu);
-
-	/*
-	 * Grab the eventfs_mutex to consistent value from ti->private.
-	 * This s
-	 */
 	mutex_lock(&eventfs_mutex);
-	ei = READ_ONCE(ti->private);
-	if (ei && !ei->is_freed)
-		ei_dentry = READ_ONCE(ei->dentry);
-	mutex_unlock(&eventfs_mutex);
 
-	if (!ei || !ei_dentry)
-		goto out;
+	ei = ti->private;
+	if (!ei || ei->is_freed)
+		goto enoent;
 
-	data = ei->data;
-
-	list_for_each_entry_srcu(ei_child, &ei->children, list,
-				 srcu_read_lock_held(&eventfs_srcu)) {
+	list_for_each_entry(ei_child, &ei->children, list) {
 		if (strcmp(ei_child->name, name) != 0)
 			continue;
-		ret = simple_lookup(dir, dentry, flags);
-		if (IS_ERR(ret))
-			goto out;
-		d = create_dir_dentry(ei, ei_child, ei_dentry);
-		dput(d);
+		if (ei_child->is_freed)
+			goto enoent;
+		lookup_dir_entry(dentry, ei, ei_child);
 		goto out;
 	}
 
-	for (i = 0; i < ei->nr_entries; i++) {
-		entry = &ei->entries[i];
-		if (strcmp(name, entry->name) == 0) {
-			void *cdata = data;
-			mutex_lock(&eventfs_mutex);
-			/* If ei->is_freed, then the event itself may be too */
-			if (!ei->is_freed)
-				r = entry->callback(name, &mode, &cdata, &fops);
-			else
-				r = -1;
-			mutex_unlock(&eventfs_mutex);
-			if (r <= 0)
-				continue;
-			ret = simple_lookup(dir, dentry, flags);
-			if (IS_ERR(ret))
-				goto out;
-			d = create_file_dentry(ei, i, ei_dentry, name, mode, cdata, fops);
-			dput(d);
-			break;
-		}
+	for (int i = 0; i < ei->nr_entries; i++) {
+		void *data;
+		umode_t mode;
+		const struct file_operations *fops;
+		const struct eventfs_entry *entry = &ei->entries[i];
+
+		if (strcmp(name, entry->name) != 0)
+			continue;
+
+		data = ei->data;
+		if (entry->callback(name, &mode, &data, &fops) <= 0)
+			goto enoent;
+
+		lookup_file_dentry(dentry, ei, i, mode, data, fops);
+		goto out;
 	}
+
+ enoent:
+	/* Nothing found? */
+	d_add(dentry, NULL);
+
  out:
-	srcu_read_unlock(&eventfs_srcu, idx);
-	return ret;
+	mutex_unlock(&eventfs_mutex);
+	return NULL;
 }
 
 /*
-- 
2.43.0.5.g38fb137bdb


[-- Attachment #4: 0001-tracefs-avoid-using-the-ei-dentry-pointer-unnecessar.patch --]
[-- Type: text/x-patch, Size: 3561 bytes --]

From 617b62d5af0913e10701b301706a09fc527e4df9 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Sat, 27 Jan 2024 13:27:01 -0800
Subject: [PATCH 1/5] tracefs: avoid using the ei->dentry pointer unnecessarily

The eventfs_find_events() code tries to walk up the tree to find the
event directory that a dentry belongs to, in order to then find the
eventfs inode that is associated with that event directory.

However, it uses an odd combination of walking the dentry parent,
looking up the eventfs inode associated with that, and then looking up
the dentry from there.  Repeat.

But the code shouldn't have back-pointers to dentries in the first
place, and it should just walk the dentry parenthood chain directly.

Similarly, 'set_top_events_ownership()' looks up the dentry from the
eventfs inode, but the only reason it wants a dentry is to look up the
superblock in order to look up the root dentry.

But it already has the real filesystem inode, which has that same
superblock pointer.  So just pass in the superblock pointer using the
information that's already there, instead of looking up extraneous data
that is irrelevant.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
 fs/tracefs/event_inode.c | 26 ++++++++++++--------------
 1 file changed, 12 insertions(+), 14 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index 1c3dd0ad4660..2d128bedd654 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -156,33 +156,30 @@ static int eventfs_set_attr(struct mnt_idmap *idmap, struct dentry *dentry,
 	return ret;
 }
 
-static void update_top_events_attr(struct eventfs_inode *ei, struct dentry *dentry)
+static void update_top_events_attr(struct eventfs_inode *ei, struct super_block *sb)
 {
-	struct inode *inode;
+	struct inode *root;
 
 	/* Only update if the "events" was on the top level */
 	if (!ei || !(ei->attr.mode & EVENTFS_TOPLEVEL))
 		return;
 
 	/* Get the tracefs root inode. */
-	inode = d_inode(dentry->d_sb->s_root);
-	ei->attr.uid = inode->i_uid;
-	ei->attr.gid = inode->i_gid;
+	root = d_inode(sb->s_root);
+	ei->attr.uid = root->i_uid;
+	ei->attr.gid = root->i_gid;
 }
 
 static void set_top_events_ownership(struct inode *inode)
 {
 	struct tracefs_inode *ti = get_tracefs(inode);
 	struct eventfs_inode *ei = ti->private;
-	struct dentry *dentry;
 
 	/* The top events directory doesn't get automatically updated */
 	if (!ei || !ei->is_events || !(ei->attr.mode & EVENTFS_TOPLEVEL))
 		return;
 
-	dentry = ei->dentry;
-
-	update_top_events_attr(ei, dentry);
+	update_top_events_attr(ei, inode->i_sb);
 
 	if (!(ei->attr.mode & EVENTFS_SAVE_UID))
 		inode->i_uid = ei->attr.uid;
@@ -235,8 +232,10 @@ static struct eventfs_inode *eventfs_find_events(struct dentry *dentry)
 
 	mutex_lock(&eventfs_mutex);
 	do {
-		/* The parent always has an ei, except for events itself */
-		ei = dentry->d_parent->d_fsdata;
+		// The parent is stable because we do not do renames
+		dentry = dentry->d_parent;
+		// ... and directories always have d_fsdata
+		ei = dentry->d_fsdata;
 
 		/*
 		 * If the ei is being freed, the ownership of the children
@@ -246,12 +245,11 @@ static struct eventfs_inode *eventfs_find_events(struct dentry *dentry)
 			ei = NULL;
 			break;
 		}
-
-		dentry = ei->dentry;
+		// Walk upwards until you find the events inode
 	} while (!ei->is_events);
 	mutex_unlock(&eventfs_mutex);
 
-	update_top_events_attr(ei, dentry);
+	update_top_events_attr(ei, dentry->d_sb);
 
 	return ei;
 }
-- 
2.43.0.5.g38fb137bdb


[-- Attachment #5: 0002-eventfsfs-initialize-the-tracefs-inode-properly.patch --]
[-- Type: text/x-patch, Size: 2548 bytes --]

From fd17eaceb3583c1c84845afce3e1560ba6a15032 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 29 Jan 2024 11:06:32 -0800
Subject: [PATCH 2/5] eventfsfs: initialize the tracefs inode properly

The tracefs-specific fields in the inode were not initialized before the
inode was exposed to others through the dentry with 'd_instantiate()'.

And the ->flags file was initialized incorrectly with a '|=', when the
old value was stale.  It should have just been a straight assignment.

Move the field initializations up to before the d_instantiate, and fix
the use of uninitialized data.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
 fs/tracefs/event_inode.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index 2d128bedd654..c0d977e6c0f2 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -328,7 +328,9 @@ static struct dentry *create_file(const char *name, umode_t mode,
 	inode->i_ino = EVENTFS_FILE_INODE_INO;
 
 	ti = get_tracefs(inode);
-	ti->flags |= TRACEFS_EVENT_INODE;
+	ti->flags = TRACEFS_EVENT_INODE;
+	ti->private = NULL;			// Directories have 'ei', files not
+
 	d_instantiate(dentry, inode);
 	fsnotify_create(dentry->d_parent->d_inode, dentry);
 	return eventfs_end_creating(dentry);
@@ -367,7 +369,8 @@ static struct dentry *create_dir(struct eventfs_inode *ei, struct dentry *parent
 	inode->i_ino = eventfs_dir_ino(ei);
 
 	ti = get_tracefs(inode);
-	ti->flags |= TRACEFS_EVENT_INODE;
+	ti->flags = TRACEFS_EVENT_INODE;
+	ti->private = ei;
 
 	inc_nlink(inode);
 	d_instantiate(dentry, inode);
@@ -513,7 +516,6 @@ create_file_dentry(struct eventfs_inode *ei, int idx,
 static void eventfs_post_create_dir(struct eventfs_inode *ei)
 {
 	struct eventfs_inode *ei_child;
-	struct tracefs_inode *ti;
 
 	lockdep_assert_held(&eventfs_mutex);
 
@@ -523,9 +525,6 @@ static void eventfs_post_create_dir(struct eventfs_inode *ei)
 				 srcu_read_lock_held(&eventfs_srcu)) {
 		ei_child->d_parent = ei->dentry;
 	}
-
-	ti = get_tracefs(ei->dentry->d_inode);
-	ti->private = ei;
 }
 
 /**
@@ -943,7 +942,7 @@ struct eventfs_inode *eventfs_create_events_dir(const char *name, struct dentry
 	INIT_LIST_HEAD(&ei->list);
 
 	ti = get_tracefs(inode);
-	ti->flags |= TRACEFS_EVENT_INODE | TRACEFS_EVENT_TOP_INODE;
+	ti->flags = TRACEFS_EVENT_INODE | TRACEFS_EVENT_TOP_INODE;
 	ti->private = ei;
 
 	inode->i_mode = S_IFDIR | S_IRWXU | S_IRUGO | S_IXUGO;
-- 
2.43.0.5.g38fb137bdb


[-- Attachment #6: 0004-eventfs-remove-unused-d_parent-pointer-field.patch --]
[-- Type: text/x-patch, Size: 1854 bytes --]

From 10edbe91e1ef0c8d3bf0be053662d2bb4fa1a6b0 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 29 Jan 2024 16:07:33 -0800
Subject: [PATCH 4/5] eventfs: remove unused 'd_parent' pointer field

It's never used

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
 fs/tracefs/event_inode.c | 4 +---
 fs/tracefs/internal.h    | 2 --
 2 files changed, 1 insertion(+), 5 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index ad11063bdd53..1d0102bfd7da 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -685,10 +685,8 @@ struct eventfs_inode *eventfs_create_dir(const char *name, struct eventfs_inode
 	INIT_LIST_HEAD(&ei->list);
 
 	mutex_lock(&eventfs_mutex);
-	if (!parent->is_freed) {
+	if (!parent->is_freed)
 		list_add_tail(&ei->list, &parent->children);
-		ei->d_parent = parent->dentry;
-	}
 	mutex_unlock(&eventfs_mutex);
 
 	/* Was the parent freed? */
diff --git a/fs/tracefs/internal.h b/fs/tracefs/internal.h
index 91c2bf0b91d9..8f38740bfb5b 100644
--- a/fs/tracefs/internal.h
+++ b/fs/tracefs/internal.h
@@ -35,7 +35,6 @@ struct eventfs_attr {
  * @name:	the name of the directory to create
  * @children:	link list into the child eventfs_inode
  * @dentry:     the dentry of the directory
- * @d_parent:   pointer to the parent's dentry
  * @d_children: The array of dentries to represent the files when created
  * @entry_attrs: Saved mode and ownership of the @d_children
  * @attr:	Saved mode and ownership of eventfs_inode itself
@@ -50,7 +49,6 @@ struct eventfs_inode {
 	const char			*name;
 	struct list_head		children;
 	struct dentry			*dentry; /* Check is_freed to access */
-	struct dentry			*d_parent;
 	struct dentry			**d_children;
 	struct eventfs_attr		*entry_attrs;
 	struct eventfs_attr		attr;
-- 
2.43.0.5.g38fb137bdb