Re: [GIT PULL] fuse update for 6.19

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

On Fri, Dec 05, 2025 at 03:47:50PM -0800, Linus Torvalds wrote:
> On Thu, 4 Dec 2025 at 00:25, Miklos Szeredi <miklos@szeredi.hu> wrote:
> >
> > The stale dentry cleanup has a patch touching dcache.c: this extracts
> > a helper from d_prune_aliases() that puts the unused dentry on a
> > dispose list.  Export this and shrink_dentry_list() to modules.
> 
> Is that
> 
>         spin_lock(&dentry->d_lock);
>         if (!dentry->d_lockref.count)
>                 to_shrink_list(dentry, dispose);
>         spin_unlock(&dentry->d_lock);
> 
> thing possibly hot, and count might be commonly non-zero?
> 
> Because it's possible that we could just make it a lockref operation
> where we atomically don't take the lock if the count is non-zero so
> that we don't unnecessarily move cachelines around...
> 
> IOW, some kind of "lockref_lock_if_zero()" pattern?
> 
> I have no idea what the fuse dentry lifetime patterns might be, maybe
> this is a complete non-issue...

Far more interesting question, IMO, is what's to prevent memory
pressure from evicting the damn argument right under us.

AFAICS, fuse_dentry_tree_work() calls that thing with no locks held.
The one and only reason why that's OK in d_prune_aliases() is ->i_lock
held over that thing - that's enough to prevent eviction.  I don't
see anything to serve the same purpose here.