changing d_parent

changing d_parent

[Jerome de Vivie]

I would like to change the "d_parent" field of a dentry. This is for
shrinking entry equivalent to a "./" in my filesystem namespace.

The dentry has just been allocated and has no child. I'm new to the 
dcache and i would like to know if thoses lines are is safe:

    spin_lock(&dcache_lock);
    list_del(&dentry->d_child);
    dput(dentry->d_parent);
    dentry->d_parent = dget(newparent);
    list_add(&dentry->d_child, &newparent->d_subdirs);
    spin_unlock(&dcache_lock);

kind regards,

j.

-- 
Jérôme de Vivie
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Re: changing d_parent

[Maneesh Soni]

On Sun, May 18, 2003 at 06:14:29PM +0000, Jerome de Vivie wrote:
> 
> I would like to change the "d_parent" field of a dentry. This is for
> shrinking entry equivalent to a "./" in my filesystem namespace.
> 
> The dentry has just been allocated and has no child. I'm new to the
> dcache and i would like to know if thoses lines are is safe:
> 
>     spin_lock(&dcache_lock);
>     list_del(&dentry->d_child);
>     dput(dentry->d_parent);
      ^^^^^^^^^^^^^^^^^^^^^^^
need to release dcache_lock() before calling dput(), as dput() also
takes dcache_lock(). Probably you can save this ->d_parent in some 
local variable and then dput() it after you have done with dcache_lock().

>     dentry->d_parent = dget(newparent);
>     list_add(&dentry->d_child, &newparent->d_subdirs);
>     spin_unlock(&dcache_lock);

Now, if you are doing this on 2.5 kernel, then you have to take the 
per dentry lock for the dentry also. some thing like this

	struct dentry * old_parent;

	spin_lock(&dcache_lock);
	spin_lock(&dentry->d_lock);
	list_del(&dentry->d_child);
	old_parent = dentry->d_parent;
	dentry->d_parent = dget(newparent);
	list_add(&dentry->d_child, &newparent->d_subdirs);
	spin_unlock(&dentry->d_lock);
	spin_unlock(&dcache_lock);
	dput(old_parent);	

This is just a sample after guessing what you are doing.

Maneesh


-- 
Maneesh Soni
IBM Linux Technology Center, 
IBM India Software Lab, Bangalore.
Phone: +91-80-5044999 email: maneesh@in.ibm.com
http://lse.sourceforge.net/

Re: changing d_parent

[Jan Hudec]

On Sun, May 18, 2003 at 07:42:29PM +0200, Jerome de Vivie wrote:
> 
> I would like to change the "d_parent" field of a dentry. This is for
> shrinking entry equivalent to a "./" in my filesystem namespace.
> 
> The dentry has just been allocated and has no child. I'm new to the 
> dcache and i would like to know if thoses lines are is safe:
> 
>     spin_lock(&dcache_lock);
>     list_del(&dentry->d_child);
>     dput(dentry->d_parent);
>     dentry->d_parent = dget(newparent);
>     list_add(&dentry->d_child, &newparent->d_subdirs);
>     spin_unlock(&dcache_lock);

One thing that comes to mind, the dentry must not be hashed and you need
to hash it at some point (d_rehash()) (IIRC it does not work on hashed
dentries).

Another thing is, that dput may lock dcache_lock, so it must be only
called after you release it.

Also note the difference of dget and dget_locked. None of them locks
dcache_lock, but dget only gets a dentry you already hold (remember, you
are not expected to hold dcache_lock), while dget_locked can get
a dentry you just found in child list/hash.

I wonder what you are really doing. You should never ever need to mess
with . and .., since lookup deals with them for you.

-------------------------------------------------------------------------------
						 Jan 'Bulb' Hudec <bulb@ucw.cz>