On 01-Jul-2023, at 7:24 PM, Steven Rostedt [off-list ref] wrote:

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"eventfs: Implement eventfs dir creation functions"

On Thu,  1 Jun 2023 14:30:06 +0530
Ajay Kaher [off-list ref] wrote:

quoted

Adding eventfs_file structure which will hold properties of file or dir.

Adding following functions to add dir in eventfs:

eventfs_create_events_dir() directly creates events dir with-in

                       "within" is a proper word.

quoted

tracing folder.

eventfs_add_subsystem_dir() adds the information of subsystem_dir to
eventfs and dynamically creates subsystem_dir as and when requires.

 "as and when requires" does not make sense.

quoted

eventfs_add_dir() adds the information of dir (which is with-in

  "within"

quoted

subsystem_dir) to eventfs and dynamically creates these dir as
and when requires.

I'm guessing you want to say:

       eventfs_add_dir() adds the information of the dir, within a
       subsystem_dir, to eventfs and dynamically creates these
       directories when they are accessed.

quoted

Signed-off-by: Ajay Kaher <redacted>
Co-developed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Tested-by: Ching-lin Yu <redacted>
Reported-by: kernel test robot <redacted>
Link: https://lore.kernel.org/oe-lkp/202305051619.9a469a9a-yujie.liu@intel.com (local)
---
fs/tracefs/Makefile      |   1 +
fs/tracefs/event_inode.c | 272 +++++++++++++++++++++++++++++++++++++++
include/linux/tracefs.h  |  29 +++++
kernel/trace/trace.h     |   1 +
4 files changed, 303 insertions(+)
create mode 100644 fs/tracefs/event_inode.c

diff --git a/fs/tracefs/Makefile b/fs/tracefs/Makefile
index 7c35a282b..73c56da8e 100644
--- a/fs/tracefs/Makefile
+++ b/fs/tracefs/Makefile

@@ -1,5 +1,6 @@

# SPDX-License-Identifier: GPL-2.0-only
tracefs-objs := inode.o
+tracefs-objs += event_inode.o

obj-$(CONFIG_TRACING)        += tracefs.o

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
new file mode 100644
index 000000000..a48ce23c0
--- /dev/null
+++ b/fs/tracefs/event_inode.c

@@ -0,0 +1,272 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ *  event_inode.c - part of tracefs, a pseudo file system for activating tracing
+ *
+ *  Copyright (C) 2020-22 VMware Inc, author: Steven Rostedt (VMware) <rostedt@goodmis.org>
+ *  Copyright (C) 2020-22 VMware Inc, author: Ajay Kaher <akaher@vmware.com>
+ *
+ *  eventfs is used to show trace events with one set of dentries
+ *
+ *  eventfs stores meta-data of files/dirs and skip to create object of
+ *  inodes/dentries. As and when requires, eventfs will create the
+ *  inodes/dentries for only required files/directories. Also eventfs
+ *  would delete the inodes/dentries once no more requires but preserve
+ *  the meta data.
+ */
+#include <linux/fsnotify.h>
+#include <linux/fs.h>
+#include <linux/namei.h>
+#include <linux/security.h>
+#include <linux/tracefs.h>
+#include <linux/kref.h>
+#include <linux/delay.h>
+#include "internal.h"
+
+/**
+ * eventfs_dentry_to_rwsem - Return corresponding eventfs_rwsem
+ * @dentry: a pointer to dentry
+ *
+ * helper function to return crossponding eventfs_rwsem for given dentry
+ */
+static struct rw_semaphore *eventfs_dentry_to_rwsem(struct dentry *dentry)
+{
+     if (S_ISDIR(dentry->d_inode->i_mode))
+             return (struct rw_semaphore *)dentry->d_inode->i_private;
+     else
+             return (struct rw_semaphore *)dentry->d_parent->d_inode->i_private;
+}
+
+/**
+ * eventfs_down_read - acquire read lock function
+ * @eventfs_rwsem: a pointer to rw_semaphore
+ *
+ * helper function to perform read lock. Nested locking requires because
+ * lookup(), release() requires read lock, these could be called directly
+ * or from open(), remove() which already hold the read/write lock.
+ */
+static void eventfs_down_read(struct rw_semaphore *eventfs_rwsem)
+{
+     down_read_nested(eventfs_rwsem, SINGLE_DEPTH_NESTING);
+}
+
+/**
+ * eventfs_up_read - release read lock function
+ * @eventfs_rwsem: a pointer to rw_semaphore
+ *
+ * helper function to release eventfs_rwsem lock if locked
+ */
+static void eventfs_up_read(struct rw_semaphore *eventfs_rwsem)
+{
+     up_read(eventfs_rwsem);
+}
+
+/**
+ * eventfs_down_write - acquire write lock function
+ * @eventfs_rwsem: a pointer to rw_semaphore
+ *
+ * helper function to perform write lock on eventfs_rwsem
+ */
+static void eventfs_down_write(struct rw_semaphore *eventfs_rwsem)
+{
+     while (!down_write_trylock(eventfs_rwsem))
+             msleep(10);

What's this loop for? Something like that needs a very good explanation
in a comment. Loops like these are usually a sign of a workaround for a
bug in the design, or worse, simply hides an existing bug.

Yes correct, this logic is to solve deadlock:

Thread 1                             Thread 2
down_read_nested()                                 - read lock acquired
                                         down_write()     - waiting for write lock to acquire
down_read_nested()                                  - deadlock

Deadlock is because rwlock wouldn’t allow read lock to be acquired if write lock is waiting.
down_write_trylock() wouldn’t add the write lock in waiting queue, hence helps to prevent
deadlock scenario.

I was stuck with this Deadlock, tried few methods and finally borrowed from cifs, as it’s
upstreamed, tested and working in cifs, please refer:
https://elixir.bootlin.com/linux/v6.3.1/source/fs/cifs/file.c#L438

Looking further for your input. I will add explanation in v4.

quoted

+}
+
+/**
+ * eventfs_up_write - release write lock function
+ * @eventfs_rwsem: a pointer to rw_semaphore
+ *
+ * helper function to perform write lock on eventfs_rwsem
+ */
+static void eventfs_up_write(struct rw_semaphore *eventfs_rwsem)
+{
+     up_write(eventfs_rwsem);
+}
+
+static const struct file_operations eventfs_file_operations = {
+};
+
+static const struct inode_operations eventfs_root_dir_inode_operations = {
+};
+
+/**
+ * eventfs_prepare_ef - helper function to prepare eventfs_file
+ * @name: a pointer to a string containing the name of the file/directory
+ *        to create.
+ * @mode: the permission that the file should have.
+ * @fop: a pointer to a struct file_operations that should be used for
+ *        this file/directory.
+ * @iop: a pointer to a struct inode_operations that should be used for
+ *        this file/directory.
+ * @data: a pointer to something that the caller will want to get to later
+ *        on.  The inode.i_private pointer will point to this value on
+ *        the open() call.
+ *
+ * This function allocate the fill eventfs_file structure.

  "allocates and fills the" ?

quoted

+ */
+static struct eventfs_file *eventfs_prepare_ef(const char *name, umode_t mode,
+                                     const struct file_operations *fop,
+                                     const struct inode_operations *iop,
+                                     void *data)
+{
+     struct eventfs_file *ef;
+
+     ef = kzalloc(sizeof(*ef), GFP_KERNEL);
+     if (!ef)
+             return ERR_PTR(-ENOMEM);
+
+     ef->name = kstrdup(name, GFP_KERNEL);
+     if (!ef->name) {
+             kfree(ef);
+             return ERR_PTR(-ENOMEM);
+     }
+
+     if (S_ISDIR(mode)) {
+             ef->ei = kzalloc(sizeof(*ef->ei), GFP_KERNEL);
+             if (!ef->ei) {
+                     kfree(ef->name);
+                     kfree(ef);
+                     return ERR_PTR(-ENOMEM);
+             }
+             INIT_LIST_HEAD(&ef->ei->e_top_files);
+     } else {
+             ef->ei = NULL;
+     }
+
+     ef->iop = iop;
+     ef->fop = fop;
+     ef->mode = mode;
+     ef->data = data;
+     ef->dentry = NULL;
+     ef->d_parent = NULL;
+     ef->created = false;

No need for the initialization to NULL or even the false, as the
kzalloc() already did that.

quoted

+     return ef;
+}
+
+/**
+ * eventfs_create_events_dir - create the trace event structure
+ * @name: a pointer to a string containing the name of the directory to
+ *        create.

You don't need to add "a pointer" we can see it's a pointer. Just say:

* @name: The name of the directory to create

Adding more makes it confusing to read.

quoted

+ * @parent: a pointer to the parent dentry for this file.  This should be a
+ *          directory dentry if set.  If this parameter is NULL, then the
+ *          directory will be created in the root of the tracefs filesystem.
+ * @eventfs_rwsem: a pointer to rw_semaphore

Same with all the descriptions.

quoted

+ *
+ * This function creates the top of the trace event directory.
+ */
+struct dentry *eventfs_create_events_dir(const char *name,
+                                      struct dentry *parent,
+                                      struct rw_semaphore *eventfs_rwsem)

OK, I'm going to have to really look at this. Passing in a lock to the
API is just broken. We need to find a way to solve this another way.

eventfs_rwsem is a member of struct trace_array, I guess we should
pass pointer to trace_array.

I'm about to board a plane to JFK shortly, I'm hoping to play with this
while flying back.

I have replied for major concerns. All other minor I will take care in v4.

Thanks a lot for giving time to eventfs patches.

- Ajay

-- Steve

quoted

+{
+     struct dentry *dentry = tracefs_start_creating(name, parent);
+     struct eventfs_inode *ei;
+     struct tracefs_inode *ti;
+     struct inode *inode;
+
+     if (IS_ERR(dentry))
+             return dentry;
+
+     ei = kzalloc(sizeof(*ei), GFP_KERNEL);
+     if (!ei)
+             return ERR_PTR(-ENOMEM);
+     inode = tracefs_get_inode(dentry->d_sb);
+     if (unlikely(!inode)) {
+             kfree(ei);
+             tracefs_failed_creating(dentry);
+             return ERR_PTR(-ENOMEM);
+     }
+
+     init_rwsem(eventfs_rwsem);
+     INIT_LIST_HEAD(&ei->e_top_files);
+
+     ti = get_tracefs(inode);
+     ti->flags |= TRACEFS_EVENT_INODE;
+     ti->private = ei;
+
+     inode->i_mode = S_IFDIR | S_IRWXU | S_IRUGO | S_IXUGO;
+     inode->i_op = &eventfs_root_dir_inode_operations;
+     inode->i_fop = &eventfs_file_operations;
+     inode->i_private = eventfs_rwsem;
+
+     /* directory inodes start off with i_nlink == 2 (for "." entry) */
+     inc_nlink(inode);
+     d_instantiate(dentry, inode);
+     inc_nlink(dentry->d_parent->d_inode);
+     fsnotify_mkdir(dentry->d_parent->d_inode, dentry);
+     return tracefs_end_creating(dentry);
+}

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