Re: [linux-usb-devel] [PATCH] back out sysfs reference count change

Maneesh Soni
2004-04-14T13:16:12Z

On Tue, Apr 06, 2004 at 03:43:20PM +0530, Maneesh Soni wrote:
[..]
>
> o The following patch fixes the race involved between unregistering a kobject
> and simultaneously opeing a corresponding attribute file in sysfs.
>
> Ideally sysfs should take a ref. to the kobject as long as it has dentries
> referring to the kobject, but because of current limitations in
> module/kobject ref counting, sysfs's pinning of kobject leads to
> hang/delays in rmmod of certain modules. The patch uses a new flag to mark
> dentries as disconnected from a valid kobject in the process of unregistering
> the kobject. The ->open in sysfs is failed if it finds a disconnected dentry.
> Marking and checking for the flag is done under dentry->d_inode->i_sem.
>
[..]

Hi Greg / Andrew,

This problem can be solved without using the d_flags as I did previously.
Viro suggested to just check for DCACHE_UNHASHED in check_perm() and this also
solves the race. Please see the following patch instead of the previous one.

Thanks
Maneesh

PS: As you might have seen sysfs-symlinks-fix.patch the sysfs_get_kobject() is
repeated in this patch also. So, once you have decided about the
sysfs-syminks-fix.patch and this one, I can re-submit a combined one.

----------------------------------------------------------------------------

o The following patch fixes the race involved between unregistering a kobject
and simultaneously opeing a corresponding attribute file in sysfs.

o Ideally sysfs should take a ref. to the kobject as long as it has dentries
referring to the kobjects, but because of current limitations in
module/kobject ref counting, sysfs's pinning of kobject leads to
hang/delays in rmmod of certain modules. The patch checks for unhashed
dentries in check_perm() while opening a sysfs file. If the dentry is
still hashed then it goes ahead and takes the ref to kobject. This done
under the per dentry lock. It does this in the inline routine
sysfs_get_kobject(dentry).

fs/sysfs/bin.c | 2 +-
fs/sysfs/file.c | 2 +-
fs/sysfs/sysfs.h | 13 +++++++++++++
3 files changed, 15 insertions(+), 2 deletions(-)

diff -puN fs/sysfs/sysfs.h~sysfs-d_fsdata-race-fix-2 fs/sysfs/sysfs.h
--- linux-2.6.5-mm5/fs/sysfs/sysfs.h~sysfs-d_fsdata-race-fix-2 2004-04-14 14:55:26.000000000 +0530
+++ linux-2.6.5-mm5-maneesh/fs/sysfs/sysfs.h 2004-04-14 15:29:51.000000000 +0530
@@ -11,3 +11,16 @@ extern void sysfs_hash_and_remove(struct

extern int sysfs_create_subdir(struct kobject *, const char *, struct dentry **);
extern void sysfs_remove_subdir(struct dentry *);
+
+
+static inline struct kobject * sysfs_get_kobject(struct dentry * dentry)
+{
+ struct kobject * kobj = NULL;
+
+ spin_lock(&dentry->d_lock);
+ if (!d_unhashed(dentry))
+ kobj = kobject_get(dentry->d_fsdata);
+ spin_unlock(&dentry->d_lock);
+
+ return kobj;
+}
diff -puN fs/sysfs/file.c~sysfs-d_fsdata-race-fix-2 fs/sysfs/file.c
--- linux-2.6.5-mm5/fs/sysfs/file.c~sysfs-d_fsdata-race-fix-2 2004-04-14 14:55:30.000000000 +0530
+++ linux-2.6.5-mm5-maneesh/fs/sysfs/file.c 2004-04-14 14:56:17.000000000 +0530
@@ -238,7 +238,7 @@ sysfs_write_file(struct file *file, cons

static int check_perm(struct inode * inode, struct file * file)
{
- struct kobject * kobj = kobject_get(file->f_dentry->d_parent->d_fsdata);
+ struct kobject * kobj = sysfs_get_kobject(file->f_dentry->d_parent);
struct attribute * attr = file->f_dentry->d_fsdata;
struct sysfs_buffer * buffer;
struct sysfs_ops * ops = NULL;
diff -puN fs/sysfs/bin.c~sysfs-d_fsdata-race-fix-2 fs/sysfs/bin.c
--- linux-2.6.5-mm5/fs/sysfs/bin.c~sysfs-d_fsdata-race-fix-2 2004-04-14 14:55:32.000000000 +0530
+++ linux-2.6.5-mm5-maneesh/fs/sysfs/bin.c 2004-04-14 14:56:17.000000000 +0530
@@ -94,7 +94,7 @@ static ssize_t write(struct file * file,

static int open(struct inode * inode, struct file * file)
{
- struct kobject * kobj = kobject_get(file->f_dentry->d_parent->d_fsdata);
+ struct kobject * kobj = sysfs_get_kobject(file->f_dentry->d_parent);
struct bin_attribute * attr = file->f_dentry->d_fsdata;
int error = -EINVAL;

_
--
Maneesh Soni
Linux Technology Center,
IBM Software Lab, Bangalore, India
email: maneesh@in.ibm.com
Phone: 91-80-25044999 Fax: 91-80-25268553
T/L : 9243696