nfsd: validate the nfsd_serv pointer before calling svc_wake_up

nfsd_file_dispose_list_delayed can be called from the filecache
laundrette, which is shut down after the nfsd threads are shut down and
the nfsd_serv pointer is cleared. If nn->nfsd_serv is NULL then there
are no threads to wake.

Ensure that the nn->nfsd_serv pointer is non-NULL before calling
svc_wake_up in nfsd_file_dispose_list_delayed. This is safe since the
svc_serv is not freed until after the filecache laundrette is cancelled.

Reported-by: Salvatore Bonaccorso <carnil@debian.org>
Closes: https://bugs.debian.org/1093734
Fixes: ffb4025 ("nfsd: Don't leave work of closing files to a work queue")
Cc: stable@vger.kernel.org
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Author: jtlayton

fs/nfsd/filecache.c

@@ -445,11 +445,20 @@ nfsd_file_dispose_list_delayed(struct list_head *dispose)
 						struct nfsd_file, nf_gc);
 		struct nfsd_net *nn = net_generic(nf->nf_net, nfsd_net_id);
 		struct nfsd_fcache_disposal *l = nn->fcache_disposal;
+		struct svc_serv *serv;
 
 		spin_lock(&l->lock);
 		list_move_tail(&nf->nf_gc, &l->freeme);
 		spin_unlock(&l->lock);
-		svc_wake_up(nn->nfsd_serv);
+
+		/*
+		 * The filecache laundrette is shut down after the
+		 * nn->nfsd_serv pointer is cleared, but before the
+		 * svc_serv is freed.
+		 */
+		serv = nn->nfsd_serv;
+		if (serv)
+			svc_wake_up(serv);
 	}
 }