Re: [PATCH v3 03/10] eventfs: adding eventfs dir add functions

On 01-Jul-2023, at 7:24 PM, Steven Rostedt rostedt@goodmis.org wrote:

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"eventfs: Implement eventfs dir creation functions"

On Thu, 1 Jun 2023 14:30:06 +0530 Ajay Kaher akaher@vmware.com wrote:

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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.

...

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.

...

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

"within"

...

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.

...

Signed-off-by: Ajay Kaher akaher@vmware.com Co-developed-by: Steven Rostedt (VMware) rostedt@goodmis.org Signed-off-by: Steven Rostedt (VMware) rostedt@goodmis.org Tested-by: Ching-lin Yu chinglinyu@google.com Reported-by: kernel test robot lkp@intel.com Link: https://lore.kernel.org/oe-lkp/202305051619.9a469a9a-yujie.liu@intel.com

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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.

...

+}

+/**

• • 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" ?

...

• */

+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.

...

• 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.

...

• • @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.

...

• • 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

...

+{

• 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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