Re: [PATCH 1/3] nfsd: nfsd_do_file_acquire should hold rcu_read_lock while getting refs

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2022-09-30 at 19:20 +0000, Chuck Lever III wrote:
> 
> > On Sep 30, 2022, at 3:15 PM, Jeff Layton <jlayton@kernel.org> wrote:
> > 
> > nfsd_file is RCU-freed, so it's possible that one could be found that's
> > in the process of being freed and the memory recycled. Ensure we hold
> > the rcu_read_lock while attempting to get a reference on the object.
> > 
> > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> 
> IIUC, the rcu_read_lock() is held when nfsd_file_obj_cmpfn() is
> invoked. So, couldn't we just call nfsd_file_get() on @nf in
> there if it returns a match?
> 
> 

Adding side effects to the comparison function seems kind of gross. Do
you know for sure that it will only be called once when you do a search?

Also, there are other places where we search the hashtable but don't
take references. We could just make those places put the reference, but
that gets messy.

I think this is cleaner.

> > ---
> > fs/nfsd/filecache.c | 9 ++++++++-
> > 1 file changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> > index d5c57360b418..6237715bd23e 100644
> > --- a/fs/nfsd/filecache.c
> > +++ b/fs/nfsd/filecache.c
> > @@ -1077,10 +1077,12 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
> > 
> > retry:
> > 	/* Avoid allocation if the item is already in cache */
> > +	rcu_read_lock();
> > 	nf = rhashtable_lookup_fast(&nfsd_file_rhash_tbl, &key,
> > 				    nfsd_file_rhash_params);
> > 	if (nf)
> > 		nf = nfsd_file_get(nf);
> > +	rcu_read_unlock();
> > 	if (nf)
> > 		goto wait_for_construction;
> > 
> > @@ -1090,16 +1092,21 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
> > 		goto out_status;
> > 	}
> > 
> > +	rcu_read_lock();
> > 	nf = rhashtable_lookup_get_insert_key(&nfsd_file_rhash_tbl,
> > 					      &key, &new->nf_rhash,
> > 					      nfsd_file_rhash_params);
> > 	if (!nf) {
> > +		rcu_read_unlock();
> > 		nf = new;
> > 		goto open_file;
> > 	}
> > -	if (IS_ERR(nf))
> > +	if (IS_ERR(nf)) {
> > +		rcu_read_unlock();
> > 		goto insert_err;
> > +	}
> > 	nf = nfsd_file_get(nf);
> > +	rcu_read_unlock();
> > 	if (nf == NULL) {
> > 		nf = new;
> > 		goto open_file;
> > -- 
> > 2.37.3
> > 
> 
> --
> Chuck Lever
> 
> 
> 

-- 
Jeff Layton <jlayton@kernel.org>