Re: [PATCH 05/14] pidfs: adapt to rhashtable-based simple_xattrs

[Christian Brauner]

On Fri, Feb 27, 2026 at 04:16:04PM +0100, Jan Kara wrote:
> On Fri 27-02-26 16:09:15, Jan Kara wrote:
> > On Mon 16-02-26 14:32:01, Christian Brauner wrote:
> > > Adapt pidfs to use the rhashtable-based xattr path by switching from a
> > > dedicated slab cache to simple_xattrs_alloc().
> > > 
> > > Previously pidfs used a custom kmem_cache (pidfs_xattr_cachep) that
> > > allocated a struct containing an embedded simple_xattrs plus
> > > simple_xattrs_init(). Replace this with simple_xattrs_alloc() which
> > > combines kzalloc + rhashtable_init, and drop the dedicated slab cache
> > > entirely.
> > > 
> > > Use simple_xattr_free_rcu() for replaced xattr entries to allow
> > > concurrent RCU readers to finish.
> > > 
> > > Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
> > 
> > One question below:
> > 
> > > +static LLIST_HEAD(pidfs_free_list);
> > > +
> > > +static void pidfs_free_attr_work(struct work_struct *work)
> > > +{
> > > +	struct pidfs_attr *attr, *next;
> > > +	struct llist_node *head;
> > > +
> > > +	head = llist_del_all(&pidfs_free_list);
> > > +	llist_for_each_entry_safe(attr, next, head, pidfs_llist) {
> > > +		struct simple_xattrs *xattrs = attr->xattrs;
> > > +
> > > +		if (xattrs) {
> > > +			simple_xattrs_free(xattrs, NULL);
> > > +			kfree(xattrs);
> > > +		}
> > > +		kfree(attr);
> > > +	}
> > > +}
> > > +
> > > +static DECLARE_WORK(pidfs_free_work, pidfs_free_attr_work);
> > > +
> > 
> > So you bother with postponing the freeing to a scheduled work because
> > put_pid() can be called from a context where acquiring rcu to iterate
> > rhashtable would not be possible? Frankly I have hard time imagining such
> > context (where previous rbtree code wouldn't have issues as well), in
> > particular because AFAIR rcu is safe to arbitrarily nest. What am I
> > missing?
> 
> Ah, I've now found out rhashtable_free_and_destroy() can sleep and that's
> likely the reason. OK. Feel free to add:

Yeah, it was a surprise to me too. :)