Re: [autofs] [RFC PATCH]autofs4: hang and proposed fix

[Trond Myklebust <trond.myklebust@fys.uio.no>]

On Tue, 2005-11-29 at 20:16 -0500, Jeff Moyer wrote:
> ==> Regarding [autofs] [RFC PATCH]autofs4: hang and proposed fix; linuxram@us.ibm.com (Ram Pai) adds:
> 
> linuxram> Autofs4 assumes that its ->revalidate() function gets called with
> linuxram> the parent_dentry's_inode_semaphore released. This is true mostly
> linuxram> but not in one particular case.
> 
> linuxram> Process P1 calls autofs4's ->lookup(). The lookup finds that the
> linuxram> dentry does not exist. It creates a dentry and adds to the
> linuxram> cache. Releases the parent's inode's semaphore and than calls
> linuxram> ->revalidate().
> 
> linuxram> Process P2 meanwhile comes in and cached_lookup() gets called. It
> linuxram> finds the dentry in the cache and finds ->revalidate() function
> linuxram> exists. So it calls ->revalidate() holding the parent's inode's
> linuxram> semaphore.
> 
> Can't we simply fix this case?  It seems like it should be perfectly safe
> to drop the parent's i_sem before calling revalidate in cached_lookup.  In
> fact, there are comments in the NFS code that would lead one to believe
> that revalidate is not supposed to be called with the parent's i_sem held:
> 
> static int nfs_lookup_revalidate(struct dentry * dentry, struct nameidata *nd)
> {
> ...
> 	/*
> 	 * Note: we're not holding inode->i_sem and so may be racing with
> 	 * operations that change the directory. We therefore save the
> 	 * change attribute *before* we do the RPC call.
> 	 */
> 
> Can you try out a patch which does this?
> 
> -Jeff
> 
> --- linux-2.6.14/fs/namei.c.orig	2005-11-29 20:14:30.000000000 -0500
> +++ linux-2.6.14/fs/namei.c	2005-11-29 20:14:48.000000000 -0500
> @@ -332,10 +332,12 @@ static struct dentry * cached_lookup(str
>  		dentry = d_lookup(parent, name);
>  
>  	if (dentry && dentry->d_op && dentry->d_op->d_revalidate) {
> +		up(&parent->d_inode->i_sem);
>  		if (!dentry->d_op->d_revalidate(dentry, nd) && !d_invalidate(dentry)) {
>  			dput(dentry);
>  			dentry = NULL;
>  		}
> +		down(&parent->d_inode->i_sem);
>  	}
>  	return dentry;
>  }

Woah! Definitely not safe. NFS might not care, but the VFS will
certainly barf over that!

By dropping the dir->i_sem in cached_lookup() you are allowing 2
processes to allocate and lookup multiple dentries for the same file
inside __lookup_hash().

Cheers,
  Trond