On Sat, 29 Oct 2022, Chuck Lever III wrote:
>
> > On Oct 28, 2022, at 2:57 PM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> >
> > The list_lru_add and list_lru_del functions use list_empty checks to see
> > whether the object is already on the LRU. That's fine in most cases, but
> > we occasionally repurpose nf_lru after unhashing. It's possible for an
> > LRU removal to remove it from a different list altogether if we lose a
> > race.
>
> I've never seen that happen. lru field re-use is actually used in other
> places in the kernel. Shouldn't we try to find and fix such races?
>
> Wasn't the idea to reduce the complexity of nfsd_file_put ?
>
I think the nfsd filecache is different from those "other places"
because of nfsd_file_close_inode() and related code. If I understand
correctly, nfsd can remove a file from the filecache while it is still
in use. In this case it needs to be unhashed and removed from the lru -
and then added to a dispose list - while it might still be active for
some IO request.
Prior to Commit 8d0d254b15cc ("nfsd: fix nfsd_file_unhash_and_dispose")
unhash_and_dispose() wouldn't add to the dispose list unless the
refcount was one. I'm not sure how racy this test was, but it would
mean that it is unlikely for an nfsd_file to be added to the dispose list
if it was still in use.
After that commit it seems more likely that a nfsd_file will be added to
a dispose list while it is in use.
As we add/remove things to a dispose list without a lock, we need to be
careful that no other thread will add the nfsd_file to an lru at the
same time. refcounts will no longer provide that protection. Maybe we
should restore the refcount protection, or maybe we need a bit as Jeff
has added here.
NeilBrown
