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

[Greg KH <greg@kroah.com>]

On Wed, Apr 14, 2004 at 06:50:15PM +0530, Maneesh Soni wrote:
> 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.

This patch looks sane, Andrew, can you let it sit in your -mm tree for a
while to see if anything breaks with it?

thanks,

greg k-h

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

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;