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SubjectRe: What protection does sysfs_readdir have with SMP/Preemption?
On Tue, Nov 22, 2005 at 01:39:47PM -0800, Greg KH wrote:
> On Tue, Nov 22, 2005 at 04:33:22PM -0500, Steven Rostedt wrote:
> > Hi,
> >
> > I'm developing a custom kernel on top of Ingo's -rt patch. My kernel
> > makes race conditions in the vanilla kernel show up very well :-)
> >
> > I just hit a bug, actually a page fault in fs/sysfs/dir.c in
> > sysfs_readdir:
> >
> >
> >
> > for (p=q->next; p!= &parent_sd->s_children; p=p->next) {
> > struct sysfs_dirent *next;
> > const char * name;
> > int len;
> >
> > next = list_entry(p, struct sysfs_dirent,
> > s_sibling);
> > if (!next->s_element)
> > continue;
> >
> > name = sysfs_get_name(next);
> > len = strlen(name);
> > if (next->s_dentry)
> > ino = next->s_dentry->d_inode->i_ino;
> >
> > ^^^^
> > This is where I had a bad pointer reference.
> >
> > else
> > ino = iunique(sysfs_sb, 2);
> >
> > if (filldir(dirent, name, len, filp->f_pos, ino,
> > dt_type(next)) < 0)
> > return 0;
> >
> >
> > Looking at this code, I don't see anything protecting the s_dentry. For
> > example, couldn't the following happen:
> >
> > sysfs_create_dir is called, which calls create_dir. Now we create a
> > dentry with no d_inode. In sysfs_make_dirent which calls
> > sysfs_new_dirent which adds to the parents s_children. Then
> > sysfs_make_dirent sets s_dentry = dentry (the one that was just made
> > with no d_inode assigned yet). Then create_dir calls sysfs_create which
> > finally assigns the d_inode.
> >
> > So, either there is some hidden protection and my modification to the
> > kernel has caused this to bug, or we have just been lucky the whole time
> > in the vanilla kernel.
>
> I think we've been lucky :(
>
> Maneesh, any ideas?
>

The dir operation sysfs_readdir() is called under directory inode's i_sem
taken in vfs_readdir() and create_dir() also takes parent directory inode's
i_sem. So in this case I think following are the relevant steps happening
which look safe to me.

cpu 0
vfs_readdir()
down(dir inode i_sem)
sysfs_readdir(dir)
parse through dir->s_dirent s_children list
up(dir inode i_sem)


cpu 1
sysfs_create_dir()
create_dir()
down(parent dir inode i_sem)
lookup_one_len (allocates & makes the new directory dentry visible)
sysfs_make_diret()
sysfs_new_dirent()
attach the new directory s_dirent to parent's s_children list)
up(parent dir inode i_sem)


Basically, sysfs_readdir for a directory is protected against any
addition/deletion in the directory by directory inode's i_sem.

But the bad pointer reference seen in sysfs_readdir() has to be debugged.
Assumption here is that if there is a dentry attached to s_dirent, there
has to be a inode associated becuase negative dentries are not created
in sysfs. Is it possible to get some more information about the recreation
scenario. Could you enable DEBUG printks for lib/kobject.c and
drivers/base/class.c to see the events happening.


Thanks
Maneesh

--
Maneesh Soni
Linux Technology Center,
IBM India Software Labs,
Bangalore, India
email: maneesh@in.ibm.com
Phone: 91-80-25044990
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