Re: [PATCH RFC 1/6] dcache: sweep cached negative dentries to the end of list of siblings

[Al Viro <viro@zeniv.linux.org.uk>]

On Wed, Apr 14, 2021 at 03:41:10AM +0000, Al Viro wrote:

> > +	if (!d_is_tail_negative(dentry)) {
> > +		parent = lock_parent(dentry);
> > +		if (!parent)
> > +			return;
> 
> Wait a minute.  It's not a good environment for calling lock_parent().
> Who said that dentry won't get freed right under it?

[snip]

FWIW, in that state (dentry->d_lock held) we have
	* stable ->d_flags
	* stable ->d_count
	* stable ->d_inode
IOW, we can bloody well check ->d_count *before* bothering with lock_parent().
It does not get rid of the problems with lifetimes, though.  We could
do something along the lines of

	rcu_read_lock()
	if retain_dentry()
		parent = NULL
		if that dentry might need to be moved in list
			parent = lock_parent()
			// if reached __dentry_kill(), d_count() will be -128,
			// so the check below will exclude those
			if that dentry does need to be moved
				move it to the end of list
		unlock dentry and parent (if any)
		rcu_read_unlock()
		return
here, but your other uses of lock_parent() also need attention.  And
recursive call of dput() in trim_negative() (#6/6) is really asking
for trouble.