Re: [PATCH 09/12] locks: helper functions for delegation breaking

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On Tue, 9 Jul 2013 15:38:36 -0400
"J. Bruce Fields" <bfields@xxxxxxxxxx> wrote:

> On Tue, Jul 09, 2013 at 09:23:10AM -0400, Jeff Layton wrote:
> > On Wed,  3 Jul 2013 16:12:33 -0400
> > "J. Bruce Fields" <bfields@xxxxxxxxxx> wrote:
> > 
> > > From: "J. Bruce Fields" <bfields@xxxxxxxxxx>
> > > 
> > > We'll need the same logic for rename and link.
> > > 
> > > Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxx>
> > > ---
> > >  fs/namei.c         |   13 +++----------
> > >  include/linux/fs.h |   33 +++++++++++++++++++++++++++++++--
> > >  2 files changed, 34 insertions(+), 12 deletions(-)
> > > 
> > > diff --git a/fs/namei.c b/fs/namei.c
> > > index cba3db1..a9d4031 100644
> > > --- a/fs/namei.c
> > > +++ b/fs/namei.c
> > > @@ -3401,14 +3401,9 @@ int vfs_unlink(struct inode *dir, struct dentry *dentry, struct inode **delegate
> > >  	else {
> > >  		error = security_inode_unlink(dir, dentry);
> > >  		if (!error) {
> > > -			error = break_deleg(target, O_WRONLY|O_NONBLOCK);
> > > -			if (error) {
> > > -				if (error == -EWOULDBLOCK && delegated_inode) {
> > > -					*delegated_inode = target;
> > > -					ihold(target);
> > > -				}
> > > +			error = try_break_deleg(target, delegated_inode);
> > > +			if (error)
> > >  				goto out;
> > > -			}
> > >  			error = dir->i_op->unlink(dir, dentry);
> > >  			if (!error)
> > >  				dont_mount(dentry);
> > > @@ -3478,9 +3473,7 @@ exit2:
> > >  		iput(inode);	/* truncate the inode here */
> > >  	inode = NULL;
> > >  	if (delegated_inode) {
> > > -		error = break_deleg(delegated_inode, O_WRONLY);
> > > -		iput(delegated_inode);
> > > -		delegated_inode = NULL;
> > > +		error = break_deleg_wait(&delegated_inode);
> > >  		if (!error)
> > >  			goto retry_deleg;
> > >  	}
> > > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > > index f951588..c37e463 100644
> > > --- a/include/linux/fs.h
> > > +++ b/include/linux/fs.h
> > > @@ -1894,6 +1894,9 @@ extern bool our_mnt(struct vfsmount *mnt);
> > >  
> > >  extern int current_umask(void);
> > >  
> > > +extern void ihold(struct inode * inode);
> > > +extern void iput(struct inode *);
> > > +
> > >  /* /sys/fs */
> > >  extern struct kobject *fs_kobj;
> > >  
> > > @@ -1962,6 +1965,28 @@ static inline int break_deleg(struct inode *inode, unsigned int mode)
> > >  	return 0;
> > >  }
> > >  
> > > +static inline int try_break_deleg(struct inode *inode, struct inode **delegated_inode)
> > > +{
> > > +	int ret;
> > > +
> > > +	ret = break_deleg(inode, O_WRONLY|O_NONBLOCK);
> > > +	if (ret == -EWOULDBLOCK && delegated_inode) {
> > > +		*delegated_inode = inode;
> > > +		ihold(inode);
> > > +	}
> > > +	return ret;
> > > +}
> > > +
> > 
> > Actually, now that I look...
> > 
> > Suppose a vfs_unlink caller passes in a NULL delegated_inode pointer.
> > He'll get back a -EWOULDBLOCK here if there's a delegation on it.
> > Presumably he'll just treat that as a hard error and the delegation
> > would never get broken. Is that expected?
> 
> Yes.
> 
> The callers that pass in NULL are callers who should really never
> encounter a write delegation.  E.g. it might happen if you exported a
> filesystem that you're also stacking ecryptfs on top of (assuming that's
> possible).  Maybe I'm missing something, but that looks pointless.
> Erroring out seems a safe way to handle it.
> 
> It wouldn't be a problem to convert more of those callers to do the same
> retry loop, it just seems better to avoid the extra spaghetti to support
> what looks like a nutty use case.
> 
> --b.

Ok, thanks for clarifying that. I agree that those others are nutty
use-cases, at least for now. If someone were to ever allow setting of
FL_DELEGs from userland though they'll probably need to contend with
them.

-- 
Jeff Layton <jlayton@xxxxxxxxxx>
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