linux-kernel - Re: [PATCH 3/6] tracefs: dentry lookup crapectomy

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Date: Tue, 30 Jan 2024 18:55:36 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] tracefs: dentry lookup crapectomy

On Tue, 30 Jan 2024 23:26:21 +0000
Al Viro <viro@...iv.linux.org.uk> wrote:

> On Tue, Jan 30, 2024 at 11:03:52AM -0800, Linus Torvalds wrote:
> > The dentry lookup for eventfs files was very broken, and had lots of
> > signs of the old situation where the filesystem names were all created
> > statically in the dentry tree, rather than being looked up dynamically
> > based on the eventfs data structures.
> > 
> > You could see it in the naming - how it claimed to "create" dentries
> > rather than just look up the dentries that were given it.
> > 
> > You could see it in various nonsensical and very incorrect operations,
> > like using "simple_lookup()" on the dentries that were passed in, which
> > only results in those dentries becoming negative dentries.  Which meant
> > that any other lookup would possibly return ENOENT if it saw that
> > negative dentry before the data rwas then later filled in.
> > 
> > You could see it in the immesnse amount of nonsensical code that didn't
> > actually just do lookups.  
> 
> > -static struct dentry *create_file(const char *name, umode_t mode,
> > +static struct dentry *lookup_file(struct dentry *dentry,
> > +				  umode_t mode,
> >  				  struct eventfs_attr *attr,
> > -				  struct dentry *parent, void *data,
> > +				  void *data,
> >  				  const struct file_operations *fop)
> >  {
> >  	struct tracefs_inode *ti;
> > -	struct dentry *dentry;
> >  	struct inode *inode;
> >  
> >  	if (!(mode & S_IFMT))
> > @@ -307,12 +304,6 @@ static struct dentry *create_file(const char *name, umode_t mode,
> >  	if (WARN_ON_ONCE(!S_ISREG(mode)))
> >  		return NULL;
> >  
> > -	WARN_ON_ONCE(!parent);
> > -	dentry = eventfs_start_creating(name, parent);  
> 
> Used to lock the inode of parent.

Actually it's the tracefs_start_creating() locks the inode, the
eventfs_start_creating() doesn't.

-- Steve


> 
> >  	if (unlikely(!inode))
> >  		return eventfs_failed_creating(dentry);  
> 
> ... and that still unlocks it.
> 
> > @@ -331,29 +322,25 @@ static struct dentry *create_file(const char *name, umode_t mode,
> >  	ti->flags = TRACEFS_EVENT_INODE;
> >  	ti->private = NULL;			// Directories have 'ei', files not
> >  
> > -	d_instantiate(dentry, inode);
> > +	d_add(dentry, inode);
> >  	fsnotify_create(dentry->d_parent->d_inode, dentry);
> >  	return eventfs_end_creating(dentry);  
> 
> ... and so does this.
> 
> >  };  
> 
> Where has that inode_lock() gone and how could that possibly work?