Message-ID: <ECB0097D-A323-4CFC-9C9E-D4DA2AA6E662@vmware.com>
Date:   Mon, 3 Jul 2023 10:13:22 +0000
From:   Ajay Kaher <akaher@...are.com>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     "mhiramat@...nel.org" <mhiramat@...nel.org>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-trace-kernel@...r.kernel.org" 
        <linux-trace-kernel@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        Ching-lin Yu <chinglinyu@...gle.com>,
        Nadav Amit <namit@...are.com>,
        "srivatsa@...il.mit.edu" <srivatsa@...il.mit.edu>,
        Alexey Makhalov <amakhalov@...are.com>,
        Vasavi Sirnapalli <vsirnapalli@...are.com>,
        Tapas Kundu <tkundu@...are.com>,
        "er.ajay.kaher@...il.com" <er.ajay.kaher@...il.com>
Subject: Re: [PATCH v3 03/10] eventfs: adding eventfs dir add functions



> On 01-Jul-2023, at 7:24 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> !! External Email
>
> FYI, all subjects should start with a capital letter:
>
> "eventfs: Implement eventfs dir creation functions"
>
> On Thu,  1 Jun 2023 14:30:06 +0530
> Ajay Kaher <akaher@...are.com> wrote:
>
>> 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@...are.com>
>> Co-developed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
>> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
>> Tested-by: Ching-lin Yu <chinglinyu@...gle.com>
>> Reported-by: kernel test robot <lkp@...el.com>
>> Link: https://lore.kernel.org/oe-lkp/202305051619.9a469a9a-yujie.liu@intel.com
>> ---
>> 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@...dmis.org>
>> + *  Copyright (C) 2020-22 VMware Inc, author: Ajay Kaher <akaher@...are.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);
>> +}
>
> !! External Email: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender.

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